Add a pie chart for Contribution Trends

[kerthcet]

As mentioned at #999, a pie chart is intuitive about what you are doing in the past period of time. But the argue is about how to pick the time slot, one suggestion is make it optional, like last week, last month, last year, all the time. cc @ChenlingLu

[634750802]

How about a line chart:

• x axis is still month
• y axis is the percent of the type of contributions of all

[ChenlingLu]

And with the optional tab as well？One for percentage, one for the total count. Also a good design with the selectable timeline, then users will know what they are doing in the past period of time.