Add #:invert? option to error-bars

[lexi-lambda]

This makes it possible to add horizontal error bars to a horizontal histogram.

[alex-hhh]

HI @lexi-lambda , I had a look at the changes and the look good, I would like to ask you to make two changes:

• pull from master into your branch, as I have configured a CI build, and I would like to see if this works for your branch
• can you please add a test case to https://github.com/racket/plot/tree/master/plot-test/plot/tests/PRs, currently there are no automated tests, so all I would like is a small example illustrating the new #:invert? option on the error bars

Thanks,
Alex.

[alex-hhh]

I was a bit confused by the way this works, as I was expecting to simply have to set #:inverted? to #t, but it seems that one also has to swap the coordinates. This, however seems consistent with the way discrete-histogram works...

#lang racket
(require plot)
(plot-width 300)
(plot-height 300)
(list
(plot (list (function sqr 1 7)
              (error-bars (list (vector 2 4 12)
                                (vector 4 16 20)
                                (vector 6 36 10))
                          #:invert? #f)))
;; NOTE: both the invert? parameter and the coordinates need to be inverted.
(plot (list (function sqr 1 7)
              (error-bars (list (vector 4 2 12)
                                (vector 16 4 20)
                                (vector 36 6 10))
                          #:invert? #t))))