small update to address firefox bug


- There is a bug in Firefox causing the chart to crash if SVG element is not rendered (display=none).  I have added a try..catch to get around the bug.
- Also gave the line chart a little headroom in the y-axis for nicer rendering
- changed svg text 'dx' to 'x' for Firefox rendering, tested in Firefox, Chrome and Safari
- popup tooltip in line chart is colored the same as the line now
- reduced tooltip size