Google bar chart: getting -1 for underscore data values

why do I get the "-1" string displayed below, even though I submit an underscore "_" to indicate missing data value here?

My bar chart - please click to see the URL parameters

Is this a bug in Google Charts?

I suspect they use -1 internally to indicate NULL values, but because I use &chds开发者_JS百科=-2064,8053 it becomes visible. They should use a MIN-1 value instead...

Thank you! Alex

Workaround is to use a value lower than the min value in range, i.e. -2065 here or you can also use -1e9 to have a shorter string.

Here is the URL for the image below: