plot: Sort data points by time elapsed

[tsenart]

This PR makes the Plot.data method return dataPoints sorted by
their X axis value (i.e. elapsed seconds)

Previously, each series would be added discontinuously to each other,
essentially not being merge-sorted correctly.

Additionally, the output color palettes were made more intuitive. Reds for
any labels that contain the word ERROR. Greens otherwise.

Any plot with more series than the available colors will be hard to
read because the colors will cycle.

Fixes #391

[tsenart]

@mightyguava: Please take a look. This is how your data will look like after this PR. Please note that I set the log-scale to 50 in the HTML plot, but that's not the default.

[mightyguava]

Looks good. Thank you!

[zachbadgett]

IMO this PR makes the plots harder to read in some cases; maybe a similar flag to -name should be added called -color to override the default or, if that's too specific of a flag, add the ability to add labels, then the plot command can read those.

[tsenart]

@zachbadgett: What visual changes do you think would make things easier to read? Can you provide some specific examples?

[zachbadgett]

@tsenart it's when you have multiple shades of same color (e.g. two green colors) they can blend in with each other. Being able to set distinguishable colors would help with that. For an example, user wants to compare the results from 100qps and 200qps ; current functionality would put them on the chart as two different shades of green which can be hard to read. With what I'm suggesting, the user can pass which color they would want to use during the attack:

vegeta attack -name=100qps -label color=green -label error=#EE7860
vegeta attack -name=200qps -label color=blue

The labels get passed downstream, plot will read the labels it supports and renders the results with the colors specified.