Certain plot elements apply to the image as a whole, not to individual plots. PHPlot will draw these at most once per image. (That is, the element will be drawn only the first time DrawGraph() is called after the element has been set up.)

For example, the first plot on an image that has a main title will result in the main title being drawn. If any subsequent plot (using the same PHPlot instance) also sets a main title, that will be ignored.

Whether you use SetPlotAreaWorld to set the plot area data range, or you let PHPlot calculate the plot area data range, that range applies to all subsequent plots unless overridden. Even if you set a new data array, the calculated or pre-set data range from the previous plot applies. Without being told otherwise, PHPlot will not re-examine the data array to recalculate the data range. This allows you to re-use an automatically calculated data range, if you want.

If instead you want PHPlot to automatically calculate the data range for additional plots, call SetPlotAreaWorld() (with no arguments), or SetPlotAreaWorld(NULL, NULL, NULL, NULL). Either of these forms causes PHPlot to forget about a specified or calculated data range, and it will compute a new range.

Of course, you can also use SetPlotAreaWorld with parameter values, to manually set all or part of the data range for each plot. Any parameters you do not set (or specify as NULL) will be calculated based on the data array for the current plot. That is, PHPlot will forget about the previous data range once you call SetPlotAreaWorld, regardless of how many non-NULL parameters you use.

When overlaying plots, you will often want all the plots to use the same data scale, so the values can be read off of the axis. Another option is to have two separate Y scales, with one represented on the left side and one on the right side. (See Section 5.34, “Example - Overlaying Plots” for an example of overlaying plots with two different Y scales.) In some cases it may make sense to overlay plots with different scales and no separate axis, for example when using data value labels, or when the important information is the trend or shape shown by the graph rather than the actual values.

You can specify a plot area (window) with SetPlotAreaPixels or SetMarginsPixels, or you can let PHPlot calculate a plot area. Whether you set the plot area yourself, or you let PHPlot calculate it, those settings apply to all subsequent plots unless overridden.

This means that if you are doing side-by-side (tiled) plots on an image, you must use SetPlotAreaPixels or SetMarginsPixels with each plot, to set the area of the image to be used for that plot. Remember that the plot area does not include the axis labels, tick marks, or titles, so you must leave enough room between and around plots for these.

If you are doing overlay plots, you can let PHPlot calculate the plot area for the first plot, or you can specify the area with SetPlotAreaPixels or SetMarginsPixels. You need not use these for subsequent plots; PHPlot will continue to use the same window, overlaying the additional plots.

However, if you allow PHPlot to calculate the plot area, it will only use information in the first plot to determine the margins. This will not work well if subsequent plots require more margin space. For example, if the first plot has a Y axis title and tick labels only on the left side, and the second overlay plot has a Y axis title and tick labels on the right side, automatic plot area calculation will only leave enough margin space on the left side. As a result, the right side Y axis title and tick labels may fall off the image edge. To avoid this, use either SetPlotAreaPixels or SetMarginsPixels to specify large enough margins.

Tick increments are recalculated for each plot, based on the data range, unless set with SetXTickIncrement or SetYTickIncrement. 'Data range' here refers to that set with SetPlotAreaWorld, or automatically calculated. This differs from the way PHPlot handles the data range, which is not recalculated by default after the first plot.

For overlay plots, if you want to use the same tick increments, you should either set the desired tick increment (doing this for the first plot is sufficient), or make sure the data ranges match.

PHPlot defaults to drawing the dependent variable grid (usually Y), and the grid lines will be drawn at tick positions. As stated above, the tick positions by default will be recalculated for each plot, using the calculated or explicitly set data range.

For overlay plots, you generally do not want to have more than one set of grid lines in each of X and Y, or the results will be confusing. Even if your plot overlays have the same tick increments, avoid having the grid lines drawn more than once. Otherwise, the grid lines for the second plot will overlay the plotted data from the first plot. You can turn off the grid lines with SetDrawXGrid(False) and SetDrawYGrid(False).

Legend position with multiple plots works differently depending on whether the position is defaulted or set, and if set what method was used.

If you set a plot area background color with SetPlotBgColor and SetDrawPlotAreaBackground, or if you set a plot area background image with SetPlotAreaBgImage, this will be applied to each plot until disabled. This works well for side-by-side (tiled) plots, as each will get the same background by default.

If you are overlaying multiple plots on an image, setting a plot area background color or image for one plot will result in that background hiding previous plots. Therefore, you need to set up the background for the first plot, then turn it off it for the second plot. If you previously set a plot area background image, you can disable it for subsequent plots with SetPlotAreaBgImage(NULL). If you previously set and enabled a plot area background color, you can disable it for subsequent plots with SetDrawPlotAreaBackground(FALSE).

You can position the X and Y axis manually with SetXAxisPosition and SetYAxisPosition, or you can let PHPlot calculate the axis positions for you. Whether you set the positions yourself, or let PHPlot calculate them for you, those positions apply to subsequent plots unless overridden. Even if you set a new data array, PHPlot will not recalculate the axis positions unless told to.

If you want PHPlot to automatically re-calculate the X axis position for a subsequent plot, use SetXAxisPosition() (with no arguments), or SetXAxisPosition(''). To restore automatic Y axis position calculation, use SetYAxisPosition() or SetYAxisPosition('').