Allow widgets to be appended to LineGraph

OK, this seems to be working.
I've created a tiny sample test case of appending a label group to a line graph in `lineGraphTests.html` (see code below) which I'm hoping you (Leslie) will clean up with a better test.

``` javascript
var lblGrpConfig80 = {...};

var labelGroup80 = new LabelGroup(lblGrpConfig80);

cntr[n] = new SVGContainer(config[n].cntr);
lineGraph[n] = new LineGraph(config[n].lg);
	
var legLines = new Legend({...});

cntr[n].append([lineGraph[n], legLines], {topPercentOffset: 0, leftPercentOffset: 0, heightPercent: 1, widthPercent: 1});
	
lineGraph[n].append(labelGroup80);
```
Although I haven't tested it; you should be able to append to the LineGraph before you append the LineGraph to the container if you want.