Makes the option to center the steps in geom_step() work

[mastropi]

Fixes bug in PR#2365 which also closes issue #3348 where the new center option is requested.

The fix was done starting off from the forked repository by rmatkins where he had implemented the new feature requested in #3348.

The bug in PR#2365 was fixed by replacing the last line in the stairstep() function from:
data_frame(x, y, data_attr)
to:
new_data_frame(c(list(x=x, y=y), data_attr))

[mastropi]

Regarding the code coverage failures, it's not clear to me where I should add unit tests for the new functionality of geom_step() implemented in this PR...
I checked testthat/test-geom-path.R but I didn't find any tests on geom_step().

[paleolimbot]

Thanks for tackling this PR! It's very close, but needs a few changes before it can be merged. See the above comments, plus:

• Could you remove randomX.png and randomY.png?
• As you've noted, this PR needs tests. There are currently no tests for the stairstep() function, which is nicely isolated and I think could be tested quite readily. With proper testing of the stairstep() function, I don't think a visual test is needed. Something like the following in tests/testthat/test-geom-path.R would work nicely:

test_that("stairstep() errors with too few observations", {

})

test_that("stairstep() output is correct for direction = 'vh'", {
  df <- data_frame(x = 1:3, y = 1:3)
  stepped <- stairstep(df, direction = "vh")
})

test_that("stairstep() output is correct for direction = 'hv'", {
  df <- data_frame(x = 1:3, y = 1:3)
  stepped <- stairstep(df, direction = "hv")
})

test_that("stairstep() output is correct for direction = 'mid'", {
  df <- data_frame(x = 1:3, y = 1:3)
  stepped <- stairstep(df, direction = "mid")
})

[willfischer]

Looking to use this exact functionality for some production code. Is it near release?

[mastropi]

I intend to perform the suggested changes and write the pending unit tests by Sunday.

[paleolimbot]

These tests are really good... thanks for adding them! Just two details and then it looks good to merge!

[paleolimbot]

Thanks!

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