fix zero and negative value for grouped barcharts

[manuel-rubio]

Closes #91

[mindok]

Thanks @manuel-rubio - I'll bundle this up with a fix to move the axis to the correct spot too.

[manuel-rubio]

you're welcome, @mindok

I've just refactored the code to keep it simpler. I was unable (at the moment) to localise the part where the axis line is drawn. Could you shed light on where is the code in charge of that part?

[mindok]

Hi @manuel-rubio - I was thinking about that over the weekend. I think the simplest approach is to just draw a zero line and leave the axes as they are. This would be done in Contex.BarChart.to_avg - there is currently a line to create the category axis (cat_axis_svg = if plot_options.show_cat_axis, do: Axis.to_svg(category_axis), else: "") - injecting something around there would work. e.g.

    zero_line = Axis.get_zero_line_svg(category_axis)

    [
      cat_axis_svg,
      val_axis_svg,
      zero_line,
      "<g>",
      get_svg_bars(plot),
      "</g>"
    ]

This will be pretty similar to Axisget_svg_gridlines for handling orientations etc, but with the line moved to the zero point.

So in axis.ex, we could have something like:

  def get_zero_line_svg(%Axis{scale: scale} = axis) do
    # Check that scale intersects zero by looking at domain
    # If so...
   domain_to_range_fn = Scale.domain_to_range_fn(scale)
   get_svg_gridline(axis, domain_to_range_fn.(0.0))
  end

[manuel-rubio]

@mindok two things... I changed the code based on what you suggested and it's not working:

I was trying to run the tests and locally they're not working, but it's not broken because of my changes, are they working for you?

[mindok]

Hi @manuel-rubio - the original tests rely on order of map keys (not a good idea). They broke with an upgrade of Elixir. It's on the to-do list...

I'll create a gallery item for the barchart that handles the zero case and see what's going on...

Could you maybe share the code that you used to generate the chart above.

[mindok]

@manuel-rubio I made a basic bar chart with a negative value in the data based on the existing plain sample in the gallery. There are a couple of issues around labels etc that still need to be considered, and the value scale doesn't seem to reset.

Issues in the sample below:

1. Data label doesn't render in the bar - the text location needs adjusting in a similar way to the bar. Your sample looks like it works ok.
2. Value axis does not extend down far enough to show full negative value (the value should be -10.7). I'm not sure if you modified the value axis generation - your example above does it correctly.
3. Bar renders over label on axis - I think this will be fixed when 2 is fixed as the category labels will be at the bottom of the plot.

Your sample looks a lot closer - are you happy to share the code used to generate it please. If you're happy for it to appear in the docs, I'll use it as a gallery example.

[manuel-rubio]

@mindok my code is here: https://github.com/altenwald/conta/blob/main/apps/conta/lib/conta/stats.ex#L166-L196

It's in charge of giving you the data about Profits & Losses from your accounting via Telegram. It's still under development.

[manuel-rubio]

Hi @manuel-rubio - the original tests rely on order of map keys (not a good idea). They broke with an upgrade of Elixir. It's on the to-do list...

If the order isn't important you can perform a Enum.sort previously to make the comparison, or create a assert_lists, something like:

defmacro assert_lists(a, b) do
  quote do
    assert Enum.sort(unquote(a)) == Enum.sort(unquote(b))
  end
end

[Zurga]

Any news on the status of this PR? I would really like to be able to create BarCharts with negative values

[mindok]

Hi @Zurga - getting there - I'm moving house this week. I've got in my calendar to look early next week.