As Kenya said, this is a date formatting issue. FYI, in Mendix, all date values include a time component. Since there is no time specified in the spreadsheets you are importing, the time is set to midnight.
As a rule, I create a string attribute for dates used when creating charts. In your case, I would create a string variable for the X Axis labels and populate that with the date values formatted the way you want them. You can still sort by the date attribute, so that the sequence of the bars is correct, but using a string attribute provides you with control over the display and formatting of date values.
Check if the Attribute for X axis value is String. if string checks how the date formatting is done for those graphs.
You can use format date time string functions to format your date to show how you require.