Avoid division by zero error when all values are zeroes

[KonstantinShemyak]

If all values to put on the chart are zero, then the size of the largest "circle axis" is maxValue. But if the user did not supply a value via configuration option, maxValue remains 0, which causes division by zero.

Set maxValue to 1 in such case. It has no affect on the chart as this happens only when all values are zeroes.

Closes #3 .