Apply y_axis prefix and suffix in the tooltip formatter

Thank you @nikathone - committed. One thing others might want to note is that values formerly in the y-axis settings will not transfer over to the Highmap Display Settings automatically and will need to be re-entered. I think the usage of this module is low enough right now that I think this is acceptable - especially since the y-axis settings did not apply to the rendered chart anyway.

Automatically closed - issue fixed for 2 weeks with no activity.