Data interpolation makes a big difference to how charts renders. Here are all of the built-in interpolations of D3.
linear - piecewise linear segments, as in a polyline.
linear-closed - close the linear segments to form a polygon.
step - alternate between horizontal and vertical segments, as in a step function.
step-before - alternate between vertical and horizontal segments, as in a step function.
step-after - alternate between horizontal and vertical segments, as in a step function.
basis - a B-spline, with control point duplication on the ends.
basis-open - an open B-spline; may not intersect the start or end.
basis-closed - a closed B-spline, as in a loop.
bundle - equivalent to basis, except the tension parameter is used to straighten the spline.
cardinal - a Cardinal spline, with control point duplication on the ends.
cardinal-open - an open Cardinal spline; may not intersect the start or end, but will intersect other control points.
cardinal-closed - a closed Cardinal spline, as in a loop.
monotone - cubic interpolation that preserves monotonicity in y.