Pressing the Integrate button performs a numerical integration of a user defined analytical function. This function can be entered/edited in the Function text box. The numerical result of the integration is displayed in the Log Panel.
The Σ button pops-up a menu listing all built-in mathematical functions provided by QtiPlot. Pressing one of the actions from this menu copies the displayed function into the edit box at the position of the text cursor.
Clicking on the button displaying a pencil icon opens a menu which lists recently used functions. By pressing one of the menu items, the selected function is inserted into the edit text box at the current position of the cursor.
Clicking the Clear button removes the contents of the Function text box.
The Variable input field displays the name of the independent variable of the function (i.e., the variable of integration). By default it is set to x.
The third input parameter is the maximum number of Subintervals that the integration algorithm may use to integrate the function. The default value of 1000 should work in almost all circumstances.
The fourth parameter is the Tolerance limit. Smaller numbers will result in the algorithm using more subintervals. The default value of 0.01 should work for most functions.
The next two fields are the lower and upper limits of integration. Set these to whatever values you need.
The Plot Area option should be checked if you want that QtiPlot draws a plot of the integrated function on the currently active layer. A table of the plotted data can be generated by right clicking on the curve and selecting the Worksheet command.