I have added to my copy:
/** Set label for X axis in date mode: the value is always represented as yyyy-mm-dd*/
#define mpX_DATE 0x03
/** Set label for X axis in datetime mode: the value is always represented as yyyy-mm-ddThh:mm:ss*/
#define mpX_DATETIME 0x04
and looks quite presentable. I will upload code if people want. The changes are small and quite localized.
Should add: the 'x' value passed can be a time_t value to present in absolute datetime terms, or starting from zero for relative datetime.
It's a good idea. Post the code, I'll integrate it in next releases.
Just realized, I need to update the overlay info box as well to format x for all the date/time types.