On newer Macs, Metal is the preferred technology for drawing the pie graph. On some older Macs, though, Metal could cause drawing issues, in which case using the older OpenGL technology is preferred.
To disable Metal, open the Terminal (in /Applications/Utilities), copy and paste the following line:
defaults write org.desairem.Disk-Graph disableMetal true
and press Enter.
To enable Metal again, use the following line instead:
defaults delete org.desairem.Disk-Graph disableMetal