Use the 'redo labels' button.
The 'alt text' in Excel consists of 'alt title' and 'description'. These properties' original purpose is to describe the image textually so that search engines and screen readers (software used
by the visually impaired) can understand what the image is. This is only rarely used, for instance when publishing an Excel sheet in HTML to the web. Hence, we can safely use this feature for our
purposes, e.g. forassigning to an order to charts in a workbook or for storing a chart image's meta information, such as source file name.
Find more on Microsoft Office Support net.
This is a rather rare event. Typcially it occurs with workbooks that have been saved in older versions of Excel before (e.g. created in 2010, saved in 2007, reopened in 2010 --> error).
You can usually workaround this by moving your data into created workbook.