Bar Chart: Better default styling for one column

**What happened**: When using the new Bar Chart panel you can provide a bar width value which works well as it divides up the space between multiple columns. Where this doesn't work so well is when you only have one column and it puts the space only to the right hand side of the column. **What you expected to happen**: When the chart is only displaying one column it should divide up the bar width value so the one column appears centred in the graph. **How to reproduce it (as minimally and precisely as possible)**: Make a bar chart panel that has only one column. **Anything else we need to know?**: This is using the new Bar Chart panel in v8.x.x This is how it displays  This is a better display