Mutations are atomic, yet minor gridlines of 0.2 mutations are shown

[corneliusroemer]

Current Behavior
When selecting divergence as opposed to time for branch length, minor gridlines at spacings smaller than 1 mutation can be shown. This makes no sense as mutations cannot occur in units smaller than 1.

Fix Gridlines should only occur in multiples of 1 for divergence.

Source: https://nextstrain.org/groups/neherlab/ncov/21A.S.478K?c=country&dmin=2020-03-11&gmax=25439&gmin=21507&label=mlabel:21A/S.478K/G22813T&m=div&p=grid&r=division

[emmahodcroft]

My guess would be this is coming from the fact that we try to reconcile enabling two types of divergence view. One is what's used for SARS-CoV-2 (actually, was implement because of SARS-CoV-2!), where we show actual # of mutations, as you show above. However, for most other viruses we don't have such small divergences that this is useful - so we show divergence in terms of subs per site per year - and here we do want to draw very fine-grained lines if needed:

https://nextstrain.org/enterovirus/d68/genome?m=div

I believe at the moment Auspice just makes a guess as to which way to display the divergence, based on total number (but I could be misremembering here). So my guess would be we'd want to add something else to that which changes how gridlines are plotted, depending on which display is being used...

[trvrb]

This relates to a broader issue of how we calculate intervals for colors as well. Integer data may get non-integer coloring stops. Be nice to improve, but this is more of a feature than a bug fix in my eyes.