avoid crash and provide a simple solution for very small log scales

[elwerene]

When using a log scale from 2 to 8, plotters crashes as tier_1 is zero and is later used as divisor. I made a simple fix by checking for zero tier_1 and providing a simple scale which iterates the integers between both numbers.

I know this is not a complete fix of the problem, especially as a small scale e.g. 0.2 to 0.8 would not show any grid lines, but the best idea a came up with at the moment and it always prevents the crash.

Sorry I had to close (https://github.com/38/plotters/pull/115) and reopen the pull request, but I did more work and just merging master would be wrong.

[codecov[bot]]

Codecov Report

Merging #116 into master will increase coverage by 0.78%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #116      +/-   ##
==========================================
+ Coverage   66.24%   67.03%   +0.78%     
==========================================
  Files          53       53              
  Lines        5134     5129       -5     
==========================================
+ Hits         3401     3438      +37     
+ Misses       1733     1691      -42     

Impacted Files	Coverage Δ
src/coord/logarithmic.rs	0.00% <0.00%> (ø)
src/chart/builder.rs	82.31% <0.00%> (+0.49%)	:arrow_up:
src/chart/context.rs	78.66% <0.00%> (+0.66%)	:arrow_up:
src/coord/datetime.rs	81.22% <0.00%> (+0.74%)	:arrow_up:
src/coord/ranged.rs	43.47% <0.00%> (+1.08%)	:arrow_up:
src/drawing/backend_impl/cairo.rs	89.74% <0.00%> (+1.46%)	:arrow_up:
src/drawing/backend_impl/bitmap.rs	84.84% <0.00%> (+1.78%)	:arrow_up:
src/style/size.rs	78.00% <0.00%> (+2.00%)	:arrow_up:
src/drawing/backend_impl/svg.rs	75.79% <0.00%> (+2.31%)	:arrow_up:
src/style/font/font_desc.rs	50.00% <0.00%> (+2.32%)	:arrow_up:
... and 3 more

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