There is no simple means to affect the font style of the labels. The only trick I know to make the labels bigger (not bold) is to increase the overall font size using the SHIFT + CTRL + PLUS keys on your keyboard.
There is a rather complex alternative using scripting, but you would need to retrieve the data coordinates on the graph via script, assign the text label recursively, and format the text all within the script.
Best,
TS
Thierry R. Sornasse