FusionCharts can work over HTTPS (Secure Sockets Layer too) in the same way as HTTP. You can make the chart function seamlessly over HTTPS by providing chart data using any of the following JavaScript methods:

• setXMLUrl
• setJSONUrl
• setChartDataUrl
• setChartData

If however you're using pure HTML embedding (which we don’t recommend) in conjunction with dataURL method, then you must clean a few headers – in order to make the chart work over HTTPS. This is required, because loading XML/JSON files in Flash over an SSL Connection in Internet Explorer fails if the Pragma:no-cache or Cache-control:no-cache HTTP headers are set on the server header. You need to remove any such settings from your server configuration.

When using Firefox or any other browser, you would not find this error. This error occurs only in Internet Explorer.
Also, this happens only in Data URL method and not in Data String method, as in dataURL method the SWF has to download XML/JSON data.