After you have plotted a waveform, you can left-click on the waveform node-title label at the top of the graph window which will set a cursor cross-bar and with popup a new floating window with the cursor position information.
Move your pointer over the new cross-bar and a number "1" will appear. If you now left-click the number that is showing you will be able to move along the graph and position the cursor.
If you click again over the node-title label then you will get the second cursor (ie. the cursor number 2) which will give the absolute value at each cursor position and the difference between the two cursor positions in the popup information window.
To remove the cursor(s) just close the popup information window.