I guess you are using Report classes to build the reports? If so try to set $view->config->selectable_columns = [metric1, metric2] in configureView method. You then should be able to choose your metric for the evolution graph.

Hi, @sgiehl. Thank you for your response.

Yes, I'm using Report class to build my report. And I'm also using the selectable_columns option but in an action of the controller related to the report. It looks like this:

$view = $this->getLastUnitGraphAcrossPlugins($this->pluginName, FUNCTION, $columns,
$selectableColumns, '', 'VisitsSummary.getMyMetrics');

Right now I'm able to select the metrics I want in the evolution graph.

It is all good until this point. My question is: let's suppose I have two metrics that I want the evolution graph to show: nb_visits and new_metric (metric made by me). How can I achieve that when I select both metrics at the same time, the nb_visits' range shows in the main Y-Axis and the new_metric's range shows in a second (the one on the right of the evolution graph) Y-Axis?

I'm attaching an image to help myself explaining better

Thank you in advance.

I think this only works if you are going to show different units. Currently the shown units are detected based on the column names (https://github.com/piwik/piwik/blob/3.x-dev/core/Metrics.php#L237-L252)
Guess we would need to extend that a bit in order to make it possible to show custom units for specific columns

Hi, @sgiehl. Thank you very much. Your response helped me a lot! I agree with you, it would be nice to show custom units for specific columns.

Once again, thank you.