Change various plot functions to take advantage of new isaxes() function. * scripts/geometry/voronoi.m, scripts/image/imagesc.m, scripts/plot/__plt_get_axis_arg__.m, scripts/plot/axes.m, scripts/plot/cla.m, scripts/plot/colorbar.m, scripts/plot/hold.m, scripts/plot/legend.m, scripts/plot/plotmatrix.m, scripts/plot/plotyy.m, scripts/plot/private/__errplot__.m, scripts/plot/private/__plt__.m, scripts/plot/view.m: Use new isaxes function to simplify code. * scripts/plot/shading.m: Use Octave convention for spacing of parentheses. * scripts/plot/zlabel.m: Correct %!test incorrectly calling plot3 with figure handle.