If you use SetPlotAreaWorld to set both Xmin and Xmax to non-NULL values, this will fix both ends of the X axis. Similarly, setting both Ymin and Ymax to non-NULL values will fix both ends of the Y axis. If you set both ends of an axis, PHPlot will use exactly that for the plot range on that axis. This is recommended if your data has a range which is known, predictable, and/or 'natural'. For example, if your Y axis represents percentage values from 0 to 100, you might use this to fix the Y axis ends:

$plot->SetPlotAreaWorld(NULL, 0, NULL, 100);

You can use SetXTickIncrement to set a fixed X tick increment, and SetYTickIncrement to set a fixed Y tick increment. If you set a fixed tick increment along an axis, PHPlot will use exactly that value (even if it results in too many, or too few tick marks).

Another way to manually specify tick increments is with SetNumXTicks or SetNumYTicks. These set the desired number of tick intervals. PHPlot will then calculate the tick increment by dividing the plot range into exactly that many intervals. For example, if the Y axis range is 0 to 17, and you use $plot->SetNumYTicks(6) to get 6 intervals, then the tick mark spacing will be about 2.833333. This is probably not a desirable value for the tick increment. Using SetNumXTicks or SetNumYTicks to set the number of tick intervals is usually not a good idea, unless you also set the plot range with SetPlotAreaWorld.

If you set both ends of the X or Y plot range, PHPlot will position tick marks starting from the lower end of that range, by default. For example, if you set the Y axis range from 1 to 20, with a tick increment of 5, the tick marks will be at 1, 6, 11, and 16. You can set a tick anchor to change this. See Section 4.6.9, “Tick Positions” below for more information.

If you use SetPlotAreaWorld to set either Xmin or Xmax (but not both) to a non-NULL value, or you set either Ymin or Ymax (but not both) to a non-NULL value, then you are fixing one end of the plot range. PHPlot will use your fixed value for that end of the range, and calculate the other end as described below.

This may make sense if your data has a fixed lower (or upper) bound. For example, if you are plotting outdoor summer temperatures in degrees Fahrenheit, you might use something like this:

$plot->SetPlotAreaWorld(NULL, 60);

which sets Ymin to 60.

Since PHPlot positions tick marks starting from the lower limit of the X and Y plot ranges, setting Xmin or Ymin establishes the basis for tick marks along that axis. For example, if you set the lower end of the X axis range to 12, with a tick increment of 10, the tick marks along X will be at 12, 22, 32, etc. You can set a tick anchor to change this. See Section 4.6.9, “Tick Positions” below for more information.

Each end of the plot range (for both X and Y) which is not specified using SetPlotAreaWorld will be calculated by PHPlot. The algorithm used by PHPlot to calculate the ends of a plot range is not perfect, but is meant to create reasonable plots in a majority of cases.

Here is an overview of the method, using the Y axis as an example. DataMin and DataMax are the smallest and largest Y values in the data array (or the values derived from the data array, for some plot types). PlotMin and PlotMax are the calculated limits of the plot range. Remember that PHPlot only calculates PlotMax and PlotMin if they have not already been set using SetPlotAreaWorld.

The range calculation uses parameters which can be set using TuneXAutoRange and TuneYAutoRange. The parameters are described in the next sections.

Here are 3 examples showing the automatic range calculation. Each figure contains 4 plots showing the cumulative effect of the numbered steps which are detailed above: (1) initial range, (2) zero magnet, (3) end adjust / increase range, and (4) end adjust / finalize ('Tick' adjust mode).

In the first example, the data is all positive (DataMin > 0).

Note: You can see in the figure about that the small adjustment to the top of the Y range made at step (3) is completely absorbed in the adjustment to reach the tick mark at step (4). This is not always the case. If the data happens to end exactly at a tick mark, and the adjust_amount parameter is greater than zero, PHPlot will extend the range to the next tick mark after that. In a more general sense, when using 'Tick' adjustment mode, the adjustment amount represents the minimum amount of space to allow between the data maximum and the plot maximum.

In the second example, the data is both negative and positive. Note that in this case the zero magnet (step 2) has no effect, and the end adjustment (step 3) and adjust to tick (step 4) change both ends of the range.

The third example has all negative data (DataMax < 0). Zero magnet applies to the top of the range, and end adjustment applies to the bottom of the range in this case.

There are two implied independent variable cases:

If you don't use SetPlotAreaWorld to set an explicit plot range for the independent variable axis in these cases, PHPlot sets the plot range for that axis to start at 0 and end at N, where N is the number of rows in the data array. The automatic range calculation is disabled for that axis, and there is no adjustment.

For both the X axis and Y axis, PHPlot calculates a tick increment if it was not set as described above. (Even if your plot has no visible tick marks along an axis, PHPlot will calculate a tick increment for that axis, since it might be used in adjusting the plot range.)

PHPlot uses one of three methods to calculate a tick increment for a given plot range (along the X or Y axis). The methods are called 'decimal', 'binary', and 'date', and correspond to the tick_mode parameter described below. By default, PHPlot uses the 'decimal' method, unless the axis labels have been set up to use date/time labels, in which case it uses the 'date' mode. You can also directly control which method is used.

Using the decimal method, PHPlot picks the largest tick increment which is 1, 2, or 5 times a power of 10, and which divides the plot range into no fewer than the acceptable minimum number of tick intervals (see the min_ticks parameter below). The power of 10 can be negative, zero, or positive. Valid automatically-calculated tick increments for non-negative powers of 10 include 1, 2, 5, 10, 20, 50, 100, 200, 500, etc. Valid automatically-calculated tick increments for negative powers of 10 include 0.5, 0.2, 0.1, 0.05, 0.02, 0.01, etc.

For example, given the range 0 to 135, and the default min_ticks of 8, PHPlot will calculate a tick increment of 10, giving 14 intervals. A tick increment of 20 is not valid because it would result in only 7 intervals, fewer than the minimum of 8. A tick increment of 5 would result in 28 intervals; while there is no explicit maximum, PHPlot chooses the largest valid increment that produces 8 or more intervals.

When using the date method, PHPlot selects from a pre-defined list of valid tick increments. The list covers a range from 1 second up to 7 days: 1, 2, 5, 10, 15, and 30 seconds; 1, 2, 5, 10, 15, and 30 minutes, then 1, 2, 4, 8, 12, 24, 48, 96, and 168 hours. PHPlot will pick the largest increment in the list which will result in no fewer than the acceptable minimum number of tick intervals (min_ticks). If the plot area range exceeds 7 days, PHPlot switches to the 'decimal' method instead, with units of 24 hours. With the 'date' method, PHPlot never chooses an increment less than 1 second.

For example, given a date/time range which spans 1020 seconds (17 minutes), and the default min_ticks of 8, PHPlot will calculate a tick increment of 120 seconds (2 minutes), which results in 9 intervals. This is the largest increment in the list above which results in 8 or more intervals.

When using the binary method, PHPlot picks the largest tick increment which is a power of 2, and which divides the plot range into no fewer than the acceptable minimum number of tick intervals (see the min_ticks parameter below). The power of 2 can be negative, zero, or positive. Valid automatically-calculated tick increments for non-negative powers of 2 include 1, 2, 4, 8, 16, 32, etc. Valid automatically-calculated tick increments for negative powers of 2 include 1/2, 1/4, 1/8, 1/16, 1/32, etc. (Note that PHPlot will label these ticks as decimal values 0.5, 0.25, 0.125 etc., not fractions.)

The tick increment calculation uses parameters which can be set using TuneXAutoTicks and TuneYAutoTicks. The parameters are described in the next sections.

// Pick a date/time tick increment on X:
$plot->TuneXAutoTicks(NULL, 'date');

// Format X labels as time values:
$plot->SetXLabelType('time', '%H:%M:%S');
// Override default and use the decimal method to pick a tick increment:
$plot->TuneXAutoTicks(NULL, 'decimal');

// Use a power of 2 for the Y axis tick increment:
$plot->TuneYAutoTicks(NULL, 'binary');

// Valid tick increments are powers of 2 >= 1
$plot->TuneXAutoTicks(NULL, 'binary', TRUE);

This section describes how PHPlot positions the tick marks along the X and Y axis (if tick marks are enabled), and the tick label values which result. Several factors are involved:

PHPlot generally places tick marks along each axis starting from the minimum value of the plot range (PlotMin), which is the left-most position on the X axis, and the bottom-most position on the Y axis. Therefore, the value of PlotMin affects all the tick values. For example, if the tick increment is 10, and PlotMin is 12, ticks will be at 12, 22, 32, 42.

A tick anchor can be set with SetXTickAnchor or SetYTickAnchor to change tick mark positions. In the above example, with PlotMin 12, if you set a tick anchor at 10, you will get tick positions 20, 30, 40, etc. Setting the tick anchor shifts all the ticks, so one of them falls at the anchor (or would, if the axis was extended). Thus, a tick anchor can be used to 'round off' the tick values to desired numbers, when the plot range minimum (PlotMin) is not a multiple of the tick increment.

If you set the plot range minimum (PlotMin) to a fixed value with SetPlotAreaWorld, consider that this value will be used as the basis for tick marks along that axis, unless you also set a tick anchor.

If PHPlot calculates the plot range minimum using the default adjustment mode T (adjust to tick, see above), then PHPlot will adjust the range to start at a tick mark. That is, PHPlot acts as if a tick anchor was set at 0. Your tick marks will be at whole multiples of the tick increment, unless you set a tick anchor to move them somewhere else.

In the other adjustments modes - I (integer) or R (range) - PHPlot makes no attempt to correlate the plot range minimum with the tick increment. Your tick marks will probably not be located at multiples of the tick increment. In mode R (range), your tick marks will likely not even be whole numbers. As in all other cases, you can set a tick anchor to position the tick marks at the values you prefer.

Here are some examples of PHPlot-calculated plot ranges and tick positions, when no tick anchor has been set.

To summarize, setting a tick anchor to force the tick positions to align one tick at a particular value, usually 0, can be useful in the situations including:

Given incomplete or contradictory data, PHPlot will always produce a positive plot range (PlotMin < PlotMax), although the range may be somewhat arbitrary. Here are some of the regressive cases, and how PHPlot produces an automatic range for them. DataMin and DataMax mean the limits of the data from the data array; PlotMin and PlotMax are the limits of the plot range. In all cases, any values specified in SetPlotAreaWorld will not be altered by PHPlot, even if it results in a plot with no visible data points.

Note that the above range choices happen as part of the "initialization" step of automatic range calculation (see Section 4.6.3, “Automatic Range Calculation” above). That is, the selected range is then subject to the same adjustments as in other cases.