SVG: read a missing coordinate of a <line> as 0 - #2167

A <line> without one of x1, y1, x2 or y2 fails with a KeyError:

SVGObject('<svg xmlns="http://www.w3.org/2000/svg"><line stroke="black" y2="6"/></svg>')
# KeyError: 'x1'

In the SVG specification the initial value of each of the four is 0. D3 relies on that: every tick of a D3 axis is written with only y2 (or x2), as <line stroke="currentColor" y2="6"></line> (tickEnter.append("line").attr(x + "2", k * tickSizeInner) in d3-axis), so an SVG export of a D3 chart could not be converted.

ShapeBuilder.line() now reads a missing coordinate as "0", as circle() and ellipse() already do for cx and cy.

The line no x1 case of test_svg_shape_tags expected the KeyError; it now expects the line from (0, 0), and a new line only y2 case covers a D3 tick. Both fail on master. The SVG tests, black and pylint pass