Rebase values in an array to match the chart scale
Even though there is not much code to review a few remarks:


returnable has a hard coded length. Should your input array ever need to be larger you'd have to remember to make this larger as well. A simple 

var returnable = new long[count.Length];


would eliminate any need to worry about this ever again.
Given the current data constraints it's not really a problem but multiplying by max first could yield in an overflow. It might pay off to store the scaling factor as double instead (in which case it can also be pre-calculated).
Using LINQ would make the code a bit bit more concise.




Refactored method:

private long[] NormalizeDataset(long[] count, long max)
{
    double scale = max / (double)count.Max();
    return count.Select(x =&gt; x * scale).ToArray();
}


Given that scale is probably around the order of 100 and that the scaling is more or less an arbitrary choice you could just not use double for the scale.