Hi,
I'm sorry, somehow I have assumed that you are asking about exporting to PNG and not checked the CSV.
What you have observed is a default behavior of the CSV export plugin, to change it, you'll need to change one line in the whole functionality.
Take a look at the Stack Overflow topic here:
https://stackoverflow.com/questions/368 ... highcharts
If you will need further assistance, please do not hesitate to ask,
Regards