fixed the keyboard accessibiilty bug in area chart on legend click

[yush-singla]

Previous Behavior

• Keyboard focus is not landing on the particular legend on graph when selecting the particular legend Option
• The color of the vertical line in line chart didn't have the correct luminosity ratio (3:1)

New Behavior

• Keyboard focus is landing on the particular legend on graph when selecting the particular legend Option
• The color of the vertical line in the line chart has the correct luminosity ratio (3:1)
  

Related Issue(s)

• Fixes # https://uifabric.visualstudio.com/iss/_workitems/edit/8692
• Fixes # https://uifabric.visualstudio.com/iss/_workitems/edit/8471

[fabricteam]

📊 Bundle size report

_{🤖 This report was generated against b34f5f262efaeb8b1e16be2f4085a1576532ef13}

[size-auditor]

Asset size changes

Size Auditor did not detect a change in bundle size for any component!

Baseline commit: b34f5f262efaeb8b1e16be2f4085a1576532ef13 (build)

[codesandbox-ci]

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