chart: label a narrow axis in decimals

A span of 1.2 degrees rounded to whole degrees printed "15" against both the
middle and bottom rows, so the axis asserted that two rows drawn at different
heights were the same temperature. It reads as a rendering fault, and the scale
becomes useless exactly when the chart is at its most magnified.

Below a two degree span the labels carry one decimal. Two degrees is where
five rows put adjacent labels half a degree apart, which still rounds apart;
below that they collide. A decimal label that would not fit the gutter -- well
below freezing, where the minus sign costs a column -- falls back to whole
degrees, which is wrong but legible and beats shearing the column.

The gutter widens by one to hold "16.0°".

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