Fix BarChart that has negative value

[k-yokoishi]

If the data in BarChart is like below,

<BarChart
  width={width}
  height={height}
  data={[-50, -20, -2, 86, 71, 100]}
  chartConfig={chartConfig}
  style={graphStyle}
/>

currently the bar chart is rendered as follow.

It's not correct bar chart. The negative numbers are usually rendered like below, thus I fixed it in this MR.

Related issue: #93

[Hermanya]

thanks, I'll take a look

[Hermanya]

this is available in v2.5.0