A request for contribution.

[awahab07]

Hi, I like this library and appreciate your effort. One of my clients is using your library in his product and wanted some enhancements. I am working for him and made some enhancements he needed. He liked my work and insisted me to contribute back my work to the repository; that's why I am writing to you.

• I have handled the commingled positive and negative values issue for stacked line and stacked bar charts.
• Implemented a bar-line chart type.
• Implemented a scatter chart with options to make it a bubble chart.
• Implemented a gauge chart.

Kindly have a look at https://github.com/awahab07/chartlets and see if this code is worth including into this great effort. You can see examples under test directory. Thanks.