The left window shows the columns available from the project's tables that may be used for plotting. If the Show current folder only option is checked, only tables from the active folder of the project will be listed in this window. The right window lists the curves already plotted. In the case presented below, there are two tables that the Add/Remove Curves... dialog box lists for the selection of columns. You select a Y column to plot using this dialog. QtiPlot will use the column defined as X in the corresponding table for the X values.
In this dialog box, if you select one curve of the plot in the right window, you can change the columns used for X and Y with the Plot Association button. In any case, you can't mix the X values from one table with the Y values from another one. If you want to do this, you have to copy the columns into the same table.
If the curve selected is a function, you can modify it by pressing the Edit Function... button. Refer to the Add Function... dialog box for more details on functions editing.
If the Show Range option is checked, the plot range of all the curves not defined as analytical functions will be also displayed in the right window of the dialog. The plot range is the interval of points which are visible in a curve. You can show/hide data points from a selected curve, without having to delete them, by pressing the Edit Range... button, which opens a dialog allowing to edit the plot range.