The plotter (also known as the compass or protractor) is a feature of GroundSchool that allows for distance and angle to be measured on certain charts.
The important thing to remember about the plotter is that it is "free" in the sense that it's not at all set to measure angles from the vertical. Rather, just like if you had a physical plotter in your hand, you have to determine where the "zero" angle is. For example, in many types of aviation chart questions, you need to decide whether the "zero" is the up-and-down solid black lines of longitude that define zero degrees true, the zero line on a VOR compass rose that indicates 0 (360) degrees magnetic in a given area, or something else entirely.
So, with that in mind, remember that you will be drawing a V-like shape, where one segment of the "V" is the zero line and the other is the angle you are trying to measure.
Getting the plotter to work takes a little fiddling, but once you get it going you will find that it is quite a powerful tool.
Here's a step-by-step guide.
First, bring up a chart in the figure pane. Then, select the plotter tool from the toolbox at top (the icon looks like a protractor).
To draw the plotter, do this: With the cursor on some part of the chart, press and hold the left mouse button. As you move the mouse, you will see a blue line drawn from the point at which you originally pushed the left mouse button down to the current mouse position. As you move the mose also, if you have selected a chart that supports distance measurement, the distance from the original point to the mouse cursor point will be continuously updated.
Let go of the left mouse button. You will see that the line you just drawn has changed from blue to orange. Now, move the mouse again. You will now see that you are drawing a second line. As you move the mouse, you will see that the second line's distance (from the vertex) is continously updated as well. Furthermore, there is also now a display indicating the angle between the first and second lines.
When you have placed the tip of the second line where you like it, click on the left mouse button to fix the second point. Voila! A drawn plotter.
However, let's say that you don't quite like where you've drawn the plotter. No problem!
First of all, you can always right-click to erase the plotter and start again.
However, more usefully, you can also move the existing points that you have drawn or slide the whole plotter accross the chart (this is useful, for example, when getting a zero-magnetic reading from a VOR rose and then sliding this over to some place of interest.)
To move an individual point (either one of the two end-points or the vertex), place the mouse cursor over the very ends of the lines or the vertex. Press and hold (ie "click and drag") the left mouse button and move the mouse. You will see that the point you have chosen moves dynamically. Release the left mouse button to "drop" the selected point in a new location.
To slide the whole plotter, place the mouse pointer over the drawn plotter anywhere except for the two ends and the vertex. Again, left click and move the mouse. You can now slide the plotter accross the virtual desk just as if you were doing so on a chart.