The troubleshooting page has some advice about GET, but nothing for POST.
Currently I am just taking away one post attribute at a time until it is no longer broken.
Is there a better way?
Hmm, the troubleshooting page recommends resubmitting a GET request with chof=validate or pasting into the Chart Playground - is there any quick way to convert GET to POST?
Is it as simple as tweaking this?
$contextArray = array("http" => array(
"method" => "POST",
"content" => http_build_query($chart, "", "&")));