I added a "web chart" web part to a sharepoint online page from the browser interface.
Next, I decided to run the Data connection wizard to connect my chart to an excel services. In the step 3 of the wizard (Retrieve and filter data), I can preview the Data.
There comes the problem: my Date column display five-digit integers instead of dates. I've tried to change the format of the column in the source excel book to YYYY-MM-DD, is does not solve my issue.
Go into page edit mode for the web part and select Advanced Properties. Click on the link for Series and scroll down to the Data section. Set either the XValueType or YValueType from Auto to Date and click Finish.
