Add test coverage

[Fil]

nyc doesn't support ESM, but c8 does!

I'm not building this by default on test:mocha because it slows things down quite a bit (down to 32s from 24s) ((and tests are already too slow)).

So you need to run yarn test:coverage to run this. It outputs the following text report (in color):

----------------------|---------|----------|---------|---------|-----------------------------------------------------------------------------------------------------------------
File                  | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s
----------------------|---------|----------|---------|---------|-----------------------------------------------------------------------------------------------------------------
All files             |   95.54 |    89.37 |   92.78 |   95.54 |
 src                  |   97.12 |    91.47 |   96.54 |   97.12 |
  axes.js             |     100 |      100 |     100 |     100 |
  channel.js          |   97.28 |    89.18 |     100 |   97.28 | 70,72-73,157,168
  context.js          |     100 |       80 |     100 |     100 | 5
  curve.js            |   89.18 |       75 |     100 |   89.18 | 52-59
  defined.js          |     100 |      100 |     100 |     100 |
  dimensions.js       |   99.32 |    87.27 |     100 |   99.32 | 144
  facet.js            |   97.97 |    95.79 |     100 |   97.97 | 110,177-178,187
  format.js           |     100 |      100 |     100 |     100 |
  index.js            |     100 |      100 |     100 |     100 |
  legends.js          |   98.76 |    81.57 |     100 |   98.76 | 55
  mark.js             |   98.19 |    87.14 |     100 |   98.19 | 63,82-83
  marker.js           |   87.71 |       90 |    90.9 |   87.71 | 15,26,28,70-80
  math.js             |     100 |      100 |     100 |     100 |
  memoize.js          |     100 |      100 |     100 |     100 |
  options.js          |   96.12 |    93.17 |   94.11 |   96.12 | 186-187,192-194,221-222,525-538
  plot.js             |   99.86 |    94.82 |   97.05 |   99.86 | 669
  projection.js       |   96.86 |    85.56 |      96 |   96.86 | 97-98,124,128,134,144,146,148,159
  scales.js           |    94.7 |    96.21 |    90.9 |    94.7 | 264-269,272-277,280-285,315-316,371,375-377,449,470-471,502-504
  stats.js            |   93.98 |    53.33 |     100 |   93.98 | 32-39
  style.js            |   99.12 |     91.6 |     100 |   99.12 | 247-250
  symbol.js           |   98.41 |    73.33 |     100 |   98.41 | 53
  template.js         |     100 |      100 |     100 |     100 |
  time.js             |   95.09 |       84 |     100 |   95.09 | 78-80,82-84,139,150
  warnings.js         |     100 |      100 |     100 |     100 |
 src/interactions     |    67.8 |    89.13 |   63.63 |    67.8 |
  pointer.js          |    67.8 |    89.13 |   63.63 |    67.8 | 80-99,109-124,138-157,160-166,169-171
 src/legends          |     100 |    93.58 |      90 |     100 |
  ramp.js             |     100 |    94.87 |   83.33 |     100 | 62,116
  swatches.js         |     100 |     92.3 |     100 |     100 | 12,165-166
 src/marks            |   95.92 |    89.56 |   94.78 |   95.92 |
  area.js             |     100 |      100 |     100 |     100 |
  arrow.js            |   95.54 |    82.85 |     100 |   95.54 | 123-129,163,167
  auto.js             |   98.67 |       88 |     100 |   98.67 | 101,145,162,175-176
  axis.js             |   99.56 |    90.73 |   97.29 |   99.56 | 439-441
  bar.js              |     100 |    95.23 |     100 |     100 | 87,92
  bollinger.js        |   64.28 |       75 |      75 |   64.28 | 19-48
  box.js              |      75 |      100 |   85.71 |      75 | 39-59
  cell.js             |     100 |      100 |     100 |     100 |
  contour.js          |     100 |     90.9 |     100 |     100 | 29,53,153,196,209
  crosshair.js        |   98.16 |      100 |   88.88 |   98.16 | 17-18
  delaunay.js         |   96.06 |    89.47 |     100 |   96.06 | 133-138,255-260
  density.js          |     100 |    84.61 |     100 |     100 | 30,43,84,107-110,152,165,173
  dot.js              |     100 |    91.93 |     100 |     100 | 61,72,95-96
  frame.js            |     100 |      100 |     100 |     100 |
  geo.js              |   97.93 |    83.33 |   77.77 |   97.93 | 75-76
  hexgrid.js          |     100 |      100 |     100 |     100 |
  image.js            |   97.74 |    84.28 |     100 |   97.74 | 124-126
  line.js             |   96.46 |       90 |     100 |   96.46 | 49-50,92-93
  linearRegression.js |   88.43 |    90.62 |   69.23 |   88.43 | 85-86,88-96,98-101,126-130
  link.js             |   97.29 |     82.6 |     100 |   97.29 | 101,108-109
  raster.js           |    99.6 |    92.14 |     100 |    99.6 | 259,267
  rect.js             |     100 |    97.29 |     100 |     100 | 101
  rule.js             |   98.58 |       88 |     100 |   98.58 | 138-139
  text.js             |     100 |    96.98 |     100 |     100 | 88,137,282-285,419
  tick.js             |     100 |      100 |     100 |     100 |
  tip.js              |   71.59 |    61.03 |   53.84 |   71.59 | 138,140,197-200,205-208,220-244,265-271,273-294,296-319,331-335,337-341
  tree.js             |     100 |     64.7 |     100 |     100 | 39,48-78,87
  vector.js           |      97 |    94.59 |     100 |      97 | 61-63,113-114
 src/scales           |   98.77 |    90.66 |    92.3 |   98.77 |
  diverging.js        |   93.93 |    88.57 |   68.75 |   93.93 | 56-57,139-140,148,151-152,159,162-163
  index.js            |     100 |      100 |     100 |     100 |
  ordinal.js          |   99.23 |    95.16 |     100 |   99.23 | 116
  quantitative.js     |   99.69 |    87.41 |     100 |   99.69 | 315
  schemes.js          |     100 |    93.87 |     100 |     100 | 186,203,265
  temporal.js         |     100 |      100 |     100 |     100 |
 src/transforms       |   92.73 |    85.16 |   86.99 |   92.73 |
  basic.js            |   86.42 |    81.94 |   86.66 |   86.42 | 63-64,102-107,116-126
  bin.js              |   92.42 |    88.63 |   81.81 |   92.42 | 275-276,281-282,309,311,313,315,343,345,347,349,351,353,423-434,449-450,455-456,461-462,467-468,473-474
  centroid.js         |     100 |      100 |     100 |     100 |
  dodge.js            |   95.86 |    89.39 |   86.66 |   95.86 | 26-27,32,42-43,51
  group.js            |   96.47 |    84.56 |   90.24 |   96.47 | 248,266,283-285,307-310,362-364,371-372
  hexbin.js           |     100 |    86.66 |     100 |     100 | 36-45,53,72-73,75,88
  identity.js         |     100 |      100 |     100 |     100 |
  inset.js            |     100 |    88.88 |     100 |     100 | 17
  interval.js         |     100 |    88.09 |     100 |     100 | 9,36-37,56-58
  map.js              |   93.33 |       76 |   88.88 |   93.33 | 58,62,65-68
  normalize.js        |   98.95 |    81.81 |     100 |   98.95 | 40
  select.js           |   83.49 |       80 |   69.23 |   83.49 | 9-15,39,43-44,55-56,59-60,66-68
  stack.js            |   89.82 |    90.15 |   81.48 |   89.82 | 16-20,37-41,134,242,244,248,250-251,254,263-265,268-276,320-325
  tree.js             |   88.92 |    66.66 |      76 |   88.92 | 130,132,148-150,157,161-163,165-167,206,208,210,220,222,224,226,228,230,234,236,238,240,251-253,255-257,267-269
  window.js           |   91.16 |    90.44 |   94.28 |   91.16 | 21-23,37,40-50,57,71,94-104
----------------------|---------|----------|---------|---------|-----------------------------------------------------------------------------------------------------------------

It also outputs coverage/lcov.info which we can use for example with the coverage-gutters extension in VSCode.

I was pleasantly surprised by our current coverage. Many of the missing lines are the ones that throw an error on invalid options.

Some thoughts/questions:

• do we need c8 as a dev dependency (I could use npx instead)
• does it make sense to activate it in CI (suggestion: comment on the coverage of checked-in files in each PR)
• I'd like to find a way to activate coverage analysis test by test, to output a matrix of (test, feature)
• do we need to clean up in prepublish?