RadHtmlChart lets you place HTML in the tooltips, but the HTML must be valid so that it does not break the rest of the page by,for example, closing another tag too early. Generally, a good approach would be to escape your HTML fully, for example, by using the HtmlEncode() method the .NET Framework offers. Note that you should do this before transforming any other special characters to HTML entities, as HtmlEncode() would transform the ampersand of the entity to its own entity, which would prevent the browser from reading it properly.
When pulling these strings from a database, you need to make sure they are prepared like this either in the database, or in the data source object you create,before passing them to the chart.
Since the displayed string is finally HTML, this means that environmental variables such as new lines must be configured for HTML—this is the
tag and not \r, \n, vbCrLf, CR, LF, chr(10) or chr(13) as used in C# and VB. In this regard, the String.Replace() method is often useful and quite straightforward to use.
The possibility to use HTML content applies only for the tooltips . RadHtmlChart labels are not HTML , but SVG text elements and therefore they do not support CSS, line breaks or other styling options that HTML/CSS offer.