The custom graph should take all the available space as long as there is only one kind of custom metric and less than 3 metrics to show.
As soon as those conditions are not fulfilled we should display two graphs one on top of another with the different metrics. One graph always display the same kind of metric. With both graph we are still limited to display only 6 metrics maximum (3 of each kind). It's also possible to display 6 metrics of the same kind on two graphs.
The zoom should remain in the same position as with one graph and should display all metrics.
The tooltips should be redesigned to not seem too big when there is two graphs displayed.