For Spotfire 7.14 and below:
Edit the visual theme: Spotfire Analyst >> View Menu >> Visual Theme >> Edit Custom Theme. Below is the reference image:
In Details tab >> Column Selector, change the font of the column selectors. Make these settings to affect the axes at all times instead of just to print. Below is the reference image:
For Spotfire 10.0 and above: Open Spotfire Analyst >> Visualizations Menu >> Canvas Styling >> Edit Custom Theme >> Details Tab >>Column Selector, change the font of the column selectors
Note: You can set the font of the axis (scale) values in a similar way, in Visualizations Menu >> Canvas Styling >> Edit Custom Theme >> Details Tab >> Visualization scales.