on chart, plot best straight line fit to data
One way is to use the “xy (scatter)” type of diagram for your data. When choosing the type of line to join the points, instead choose a “trend” line and least-squares fit.
@ve3oat: this is the answer, not a comment
If you have plotted your data in an “xy (scatter)” type of chart :
Double click on the chart to enter Edit mode;
Right click on any data point;
In the dialog box, select Insert Trend Line;
In the dialog box, under Regression Type, select Linear;
Le voilà! You can edit the visual characteristics of the trend line and you might want to remove the lines that already join the different points.