examples: Modify components to support negative bars and columns

[mhkeller]

Pretty easy fix here. On the column chart make these two adjustments:

 $: columnHeight = d => {
    return Math.abs($yGet(d) - $yScale(0) );
  };

and

y="{Math.min($yScale(0), $yGet(d))}"

Pretty similar for the bar chart component. Not sure if these need to be added to the stacked components? That seems like a weird chart type...