Thank you for your contribution.
You can always copy the file under your theme or child theme and edit it. For example, if you place the file into
wp-content/themes/YOURTHEME/templates/wallets-exchange/chart.php, then you can edit it all you want and it will override the plugin’s template. For more details on this, see the documentation or the release notes for 5.0.0.
However, you make a valid point. I can change the template so it outputs “auto” if no dimension is explicitly specified. This will go into the next release of the exchange.