I am afraid that due to limitations in SVG (it has no line breaks) this is not possible at the moment with RadHtmlChart. We have the idea logged for research and you can use this page to monitor, leave a comment on the way you would like to see things implemented or cast your vote: http://feedback.telerik.com/Project/108/Feedback/Details/38633
You can consider adding an absolutely positioned span/label/div
outside of the chart
that will hold your custom title for the time being and will be placed over it with CSS.
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