curvechart: handle missing values
curvechart: groups -> group; two colored groups in test
curvechart: curveSelect -> curvesSelect (more like pointsSelect in scatterplot)
curvechart: tool tips with d3-tip (but they look bad)

  - located on the right, but the height seems to be taken from the
    middle of the path rather than where it ends up
curvechart: remove d3-tip and indicate individual with text in top-left