Change graphviz font family to sans

[freyam]

• Tests passed
• Passes black dask / flake8 dask / isort dask

I was working on #7869 and realized that the Graphviz output would look even cleaner if we use a sans-serif font instead of the current serif font.

In this PR, I have changed the font of the Graphviz from Times-Roman to Roboto Helvetica. I have also cleaned the code in the method to_graphviz() to be more consistent in dot.py (LOW LEVEL) and highlevelgraph.py (HIGH LEVEL). Now, they look similar and it might be easier to work with this code in the future.

It follows the https://marketing.dask.org/en/latest/colors.html

(Font mentioned in image: Roboto)

Even the HTML Representation uses a Sans font, so it makes sense to have the Graph representations also follow suit.

Demo

import dask.array as da

x = da.ones((10, 10), chunks=(5, 5))
x = x + 100
x = x * 100

x.visualize()

Before

After

import dask.array as da

XL = da.ones((67552, 365941), chunks=(652, 5792))
XL2 = XL.T.astype("f4")
XC = da.ones((365941, 26), chunks=(365941, 26))
LS = da.linalg.lstsq(XC, XL2)[0]
XC_LS = XC @ LS
XLP = XL2 - XC_LS

XLP.dask.visualize(\)

Before

After

/cc @GenevieveBuckley @martindurant

[GenevieveBuckley]

My personal opinion here is the same as described in #7869 (comment)

I'm mildly supportive of the suggestion to use a sans serif font for consistency with other representations of the high level graph, but I wouldn't die on this hill if other people dislike it. Now that the font size is often a lot bigger, it certainly seems more like a bunch of headings (where sans serif is typically expected) rather than paragraph-like text.

[martindurant]

I am also +1 here, so lets merge in a day if there is no dissent.

[jrbourbeau]

Thanks @freyam!

[freyam]

This is ready to merge, if no one has any issues.

/cc @martindurant @jrbourbeau

[freyam]

💛