ImageGraph background transparent

General questions
1

hello how Can Have a transparent background with the method ImageGraph
i tried

http://localhost/news/modules/doldesign/piwik/index.php?module=API&method=ImageGraph.get&idSite=2&apiModule=VisitsSummary&apiAction=get&token_auth=anonymous&graphType=evolution&period=day&date=previous30&backgroundColor=transparent&width=500&height=250

but the background is black
image here

2

Did you tried setting backgroundColor as the same of your page’s background color?