In the sidebar on the left, there is the Create button with a drop down menu for creating new plots. Click on it, and select Cartesian Plot. The text box for editing the current equation will be focused. Replace the default text with
y = x^2
and press Enter. This will draw the plot of y = x2 in the coordinate system. Clicking on the Create button again, select Cartesian Plot, and this time enter the text
y = 5sin(x)
to get another plot.
Click on one of the lines you have just plotted. Now the crosshair becomes the color of the current plot and is attached to the it. You can use the mouse to move the crosshair along the plot. In the status bar at the bottom of the window the coordinates of the current position is displayed. Note that if the plot touches the horizontal axis the root will be displayed in the status bar, too.
Click the mouse again and the crosshair will be detached from the plot.