Checking this option will automatically produce a data plot for a map marker as you hover your mouse cursor over it. This can be a useful technique for quickly assessing the latest trends across multiple sites for the variable selected. Once the data plot has been displayed, you can keep the plot open by clicking on that marker. Otherwise, moving your mouse cursor away from the marker will automatically close the data plot window.
Alternatively, if you do not wish to have data plots popping up as you move your mouse cursor over the map, simply deselect the "show plot on hover" checkbox. While this turns off the hover action, you can still click on a marker to produce its data plot.