Get multiple siteID on Image Graph (no replies)

If this would output a single site graph with visits on it,

<img src="http://domain.com/index.php?module=API&method=ImageGraph.get&idSite=1&apiModule=VisitsSummary&apiAction=get&token_auth=<?auth_here ?>&graphType=evolution&period=day&date=previous90">

How can I output multiple site id's with each of their number of visits?
This would not work

<img src="http://domain.com/index.php?module=API&method=ImageGraph.get&idSite=1%2C2&apiModule=VisitsSummary&apiAction=get&token_auth=<?auth_here ?>&graphType=evolution&period=day&date=previous90">