Sourcerer is a third party Plugin that enables you to include PHP code in an article. You could calculate a value in PHP and then pass the result to a Plotalot chart by setting a variable that the Plotalot Plugin can read. For example:
{source}
<?php
$degreef = 123;
$degreec = round(($degreef - 32) * 5/9, 1);
$jinput = JFactory::getApplication()->input;
$jinput->set('temperature', $degreec);
?>
{/source}
{plotalot id="9" P1="_temperature"}