Let’s say you are making a Hubble Diagram with the following data:
You can simply click on the upper left data cell, hold down the mouse button, and drag to the lower right data cell. All the cells you have selected will be highlighted as shown below:
CAUTION: Do not highlight the column names. Be sure only cells containing data are selected! If you make a mistake, click outside of your selection, then try again.
If the columns that you want to select are not next to each other, select the first column, then hold down the Ctrl key as you select the second column.