Secondary axis overwrites first

@zerbash - apologies, I wasn't seeing your changes when I made my patch. I'm probably fine with either - can you take a look at them and let me know your thoughts?

I don't think checking for ['#opposite'] is the best way to determine a secondary y-axis. That's a valid setting that a user may want to apply to the default y. I think if yAxisID has been set, it's fair to assume that the target is not y:

        $target_axes = array_column($chart_definition['data']['datasets'],'yAxisID');
        $y_axis_key = in_array($child, $target_axes) ? $child : 'y';

andileco → credited nikathone → .

Thank you! Automated and manual tests passed for me.

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