add correlation plot type

[chrisamiller]

See glioviz - choose geneA, geneB, plot their relationship between samples. Could work for proteomics or expression

[chrisamiller]

• if looking at the gliovis example, don't need the density points on the side
• would be nice to have mouseovers on the points that show sample name
• could add a correlation (cor() or stats like they do)