Below is a snapshot of one of our dashboard files that we use to publish performance tables and charts on the SharePoint.
As you can see the charts are not working, but the tables are working fine. the message is: Failed to download chat or image.
To solve this issue should try changing the "Maximum Chart Size" Excel Services Trusted Location setting. By default it is 1MB, so if a chart is bigger than that, it won't render.
To change the Maximum Chart or Image Size try the following steps:
• Open Central Administration Site (need Permission)
• Click on Manage service applications Link
• Click on Excel Services Application link
• Click on Trusted File Locations Link
• select http:// sub menu row item, and select edit
• Go to Workbook Properties and change the Maximum Chart or Image Size to (as example) 10
• Click OK
• Now do IIS reset, and open the dashboard files.
Good luck 
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