Hello. I don't know why no one has done this for so many years, but I found a method for adding this indicator to the chart (and any other available ones). This is most likely the wrong approach, but it works :)
Actually, you only need to add one line

$response['metadata']['imageGraphUrl'] .= '&columns=nb_visits,nb_uniq_visitors';

at the end of the method

enrichProcessedReportIfVisitsSummaryExtendGet(&$response, $infos)

in file

../plugins/VisitsSummary/VisitsSummary.php

You can see what it looks like in my mini-plugin

Hi @kiber-io, thanks for figuring this out! If this works correctly (I haven't actually tested it), would you be willing to create a pull request for it? I believe the proper place to change the imageGraphUrl property would be in the plugins/VisitsSummary/Reports/Get.php file by adding the following method:

protected function buildReportMetadata()
{
    $reportMetadata = parent::buildReportMetadata();
    $reportMetadata['imageGraphUrl'] = // ...
    return $reportMetadata;
}

That should have the same effect (again, I haven't tested, just looked at the code a bit).

That should have the same effect

Unfortunately, no, at this stage the required key is not in the array

would you be willing to create a pull request for it?

I didn't do this, because it's hardcore and not everyone needs this indicator in the report