Bug in y-axis inference

[dan-knight]

When two branch length columns are provided, the second y-axis isn't always generated (even when the second branch itself is rendered).

Example Input

tree <- data.frame(
    parent = c(NA, 1, 2),
    length1 = c(10, 10, 100),
    length2 = c(1, 1, 0.1)
    );

plot <- SRCGrob(tree);
grid.draw(plot);