Known issue. This relates to the use of a 3rd party graphics library that we cannot modify so this is recorded as a known issue with no known current fix.
Users exporting a Dashboard to "png" format or receiving an email subscription containing an image of a dashboard may see a result where the charts differ.
This is most common in charts that are configured to view "%Change over time" or "$Change over time". The symptoms include the export or subscribed chart image showing values at 900%, "Generate PNG" error, or "No Data Available" but the dashboard displays correctly in the platform.
This is due to the reprocessing of data to create the PNG. The underlying library processes the data of the dashboard a second time attempting to calculate the percent change of a percent change resulting in divide by zero or out of bounds errors.
Workaround by using the Platform UI Dashboard as it shows the correct values.