Transition With Style
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D3 is smart enough to select an appropriate scale for the property in transition. If a transitioning element is not given an explicit starting point, D3 will use the computed style. If the end value is missing, the property will be treated as a constant. In the example above, the border stays the same, as do the top and bottom margins. The only properties that change are the ones specified below the transition().
Data Visualization with D3.js Cookbook (affiliate link)