Sparkline content isn't rendered correctly when input data is all zeros

[pridmorej]

Hello,

Firstly, thank you for providing this library, it's a really handy tool.

In SparklineFont.DrawSparkline you search the input data for min and max, however, if the data only contains zeros, max is set to zero and therefore pcy0 and pcy1 are set to NaN due to a divide by zero issue.

For example, the input data "0.0,0.0,0.0" results in the GeneratedText being as per sample below and not rendering correctly in the browser.

<span class="Sparklines"><span class="sparkline-start">&#-2147419558;</span>
 &#-2147483472;&#-2147483600;&#-2147483600;&#-2147483472;&#-2147483600;&#-2147483600;
<span class="sparkline-stop">&#-2147419558;</span></span>

I'd have a go at creating a PR, but not sure how you'd want it fixed. Any chance you could take a look please?