I just bought the User version, downloaded and tried to install on Joomla 2.58 but all I get is:

There was an error uploading this file to the server.

I uninstalled the Free version and tried again, but no luck. Ultimately I reverted to the free version, until I solve this issue, or do I have to get a refund?

Manually trying to install from the /homepages/.../tmp directory, after uploading the file, results in:

Error installing module

Please, check our FAQs
www.artio.net/faqs/joomdoc/error-when-installing-the-component

Most likely, the problem is related to a Upload Size limit set on your server.

That was indeed the problem. Had to uninstall the Free version first and then the User version installed.

However, it appears all labels in existing charts have disappeared and with new charts labels are not displayed at all, for instance with Single Series Bar 2D charts.

Hello,

Flash charts are different for free version and paid version - so data for charts from free version are not fully compatible with charts from paid version.

As you can see in our demo, labels are visible in both charts.
demo.tramin.artio.cz/joomla25/en/bar-cha...bar-2d-and-3d-charts

You probably need to modify your graphs in administration part of website and resave them. It could help.

Other communication on topic: Labels missing from chart.