Add radial and linear gradient sample files

[etimberg]

Adds a sample file that uses scriptable options to create a radial gradient for a polar area chart. Adds a second sample file that creates a linear gradient for a line chart.

Resolves at some or all of these which refer to gradients

• 2 color or gradient color in donut charts #6118
• Apply Gradients to Lines Relatively to Allow Proper Scaling #6064 (though this is not a line chart, the concept is the same)
• support different color maps #5117 (?)
• [Feature Request] Infographics (Bar) Charts #4390
• [FEATURE] - Line Chart with Colored Filled Regions (instead of a single color only) #4068

[benmccann]

Cool!