Update/scales and tests

[nkyllonen]

Currently branches off of refactor/axis-labels-and-grid-lines. Waiting for #24 to merge first before trying to merge into philosophical-rewrite.

[nkyllonen]

Now that I've thought about this again, I think: no, scales should NOT support a start option since that's purely for presentation and not part of how the data is handled.

This is a possible way to keep the start attribute within the labels, but handle it within scale_values/2 and don't let it trickle down into the Scales themselves:

@doc """
Returns scale values for rendering labels and grid lines.

Supports a `:start` option to begin iteration from a specific value within the scale.

## Example

    iex> scale_values(x_axis, step: 5)
    [~D[2023-08-01], ~D[2023-08-06], ...]
    
    iex> scale_values(x_axis, step: 5, start: ~D[2023-08-03])
    [~D[2023-08-03], ~D[2023-08-08], ...]
"""
def scale_values(%{scale: scale}, opts \\ []) do
  opts = Map.new(opts)
  {start_value, scale_opts} = Map.pop(opts, :start)
  
  values = Scale.values(scale, scale_opts)
  
  case start_value do
    nil -> 
      values
    start -> 
      # Drop values until we reach the start point
      Enum.drop_while(values, fn v -> v != start end)
  end
end

Benefits:

✅ Scale protocol stays pure (only step/ticks)
✅ Presentation layer handles filtering
✅ Works with any scale type
✅ Animated demo still works
✅ Follows design doc separation
This means removing :start from DateScale, DateTimeScale implementations and just keeping it as a scale_values/2 feature.

⚠️ 🤖 This is a Claude generated suggestion however- so it still needs to be vetted and tested.

[nkyllonen]

Now that I've thought about this again, I think: no, scales should NOT support a start option since that's purely for presentation and not part of how the data is handled.

After much back and forth, I don't see a way to remove the start option from Scales.values/2 while maintaining the ability to specify a starting value. Also, I'm coming around to realizing that Scales lie at the intersection of data and presentation. The ticks option is an example of this: that is how we specify which values within a Scale we want to render. We can only do that with the data contained within the Scale. The same can be said about having a start value: we can only set a start value with the data contained with a Scale- it can't be properly done externally. Especially in conjunction with a number of ticks.