make more `Plot` attributes mutable

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.15 :warning:

Comparison is base (ab91729) 99.80% compared to head (eebdcf1) 99.65%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #338 +/- ## ========================================== - Coverage 99.80% 99.65% -0.15% ========================================== Files 34 34 Lines 2048 2029 -19 ========================================== - Hits 2044 2022 -22 - Misses 4 7 +3 ``` | [Impacted Files](https://codecov.io/gh/JuliaPlots/UnicodePlots.jl/pull/338?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaPlots) | Coverage Δ | | |---|---|---| | [src/plot.jl](https://codecov.io/gh/JuliaPlots/UnicodePlots.jl/pull/338?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaPlots#diff-c3JjL3Bsb3Quamw=) | `100.00% <ø> (ø)` | | | [src/common.jl](https://codecov.io/gh/JuliaPlots/UnicodePlots.jl/pull/338?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaPlots#diff-c3JjL2NvbW1vbi5qbA==) | `99.57% <100.00%> (-0.02%)` | :arrow_down: | | [src/show.jl](https://codecov.io/gh/JuliaPlots/UnicodePlots.jl/pull/338?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaPlots#diff-c3JjL3Nob3cuamw=) | `100.00% <100.00%> (ø)` | | | [ext/FreeTypeExt.jl](https://codecov.io/gh/JuliaPlots/UnicodePlots.jl/pull/338?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaPlots#diff-ZXh0L0ZyZWVUeXBlRXh0Lmps) | `99.10% <0.00%> (-0.90%)` | :arrow_down: | | [src/canvas.jl](https://codecov.io/gh/JuliaPlots/UnicodePlots.jl/pull/338?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaPlots#diff-c3JjL2NhbnZhcy5qbA==) | `99.42% <0.00%> (-0.58%)` | :arrow_down: | | [src/canvas/asciicanvas.jl](https://codecov.io/gh/JuliaPlots/UnicodePlots.jl/pull/338?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaPlots#diff-c3JjL2NhbnZhcy9hc2NpaWNhbnZhcy5qbA==) | `100.00% <0.00%> (ø)` | | | [src/interface/contourplot.jl](https://codecov.io/gh/JuliaPlots/UnicodePlots.jl/pull/338?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaPlots#diff-c3JjL2ludGVyZmFjZS9jb250b3VycGxvdC5qbA==) | `100.00% <0.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaPlots). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaPlots)

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