I called the API by this:
/index.php?module=API&method=ImageGraph.get&idSite=1&token_auth=xxxxxxxxxxxxx&period=month&date=previous24&apiModule=VisitsSummary&apiAction=get&graphType=evolution&width=0&height=2
and got this fatal error message
width = 0 AND height != 0
width = 0 and height = 0 => it works fine.
Thanks for creating this issue @mathContao I can reproduce it using the same parameters 👍