hi,
What you want, need to change the model of chart tables and implementation, not easy. (i suppose the solution add chart "attribute" per "dimension" and "measure" where you can define attributes like suffix -M and multiplier/divider (similar eg. to klipfolio or other bi tools). However as Deepak suggest above you can do the trick - divide value by million in the chart source, then add to the title Amt (Mill.)
Our changes for charts and dashboards, you can find here:
We plan to commit multi-dashboard, first then later commit chart improvements.
Norbert