Tooltips are the little comments that appear when the cursor rests on some of the buttons of CalGraph. Don't click, just wait a moment. If the tooltips do not appear, they may have been turned off. Click Customize at the top of the window and then Restore Tooltips.
If you don't like them, click Customize at the top of the window and then Remove Tooltips.
For each function button, the tooltip indicates the Domain and Range.
For example, hold the cursor over the 
button, and the yellow
tooltip says Domain
= (0,infinity), Range = all. In English this says that Log is defined
for numbers running from 0 to infinity (that is the positive numbers)
and its output can be any number at all. This can be helpful if you try
to plot a graph and nothing appears, just the message Out
of Bounds. It can also suggest the problem if only part of the
graph you expect is visible.