When configuring the Dundas Web Chart, why do I need a both url AND Dir values?

Question

We use Dundas ASP.NET charts on our website. Our website runs on multiple servers and we use Dundas Ajax functionality, so we need to ensure that Dundas stores it's temporary images in a central location.

According to Dundas documentation, we can configure the chart handler as follows:

<appSettings>
<add key="ChartHttpHandler" value="Storage=file;Timeout=60;
Url=http://dundas.com/temp/;Dir=\\dundas\wwwroot\inetpub\website\temp"/&gt;
</appSettings>

My question is: why is the URL required if you specify Dir? Are they Both required? Or is Dir used and Url ignored? Or are both values supposed to point to the same location?