fix(shape): line.step.type config not working

- add d3 curveStepBefore and curveStepAfter api for check interpolate type

Ref naver#85

spec/shape.line-spec.js

@@ -26,6 +26,9 @@ describe("SHAPE LINE", () => {
 						["data3", -150, 120, 110, 140, 115, 125]
 					],
 					type: "line"
+				},
+				line : {
+					step : {}
 				}
 			};
 		});
@@ -51,6 +54,13 @@ describe("SHAPE LINE", () => {
 			expect(true).to.be.ok;
 		});
 
+		it("should change line type to step-after", () => {
+			args.line.step.type = "step-after";
+
+			expect(true).to.be.ok;
+		});
+
+
 		it("should have shape-rendering = crispedges when it's step chart", () => {
 			chart.internal.main.selectAll(".bb-line").each(function() {
 				const style = d3.select(this).style("shape-rendering").toLowerCase();

src/internals/shape.js

@@ -3,6 +3,8 @@
  * billboard.js project is licensed under the MIT license
  */
 import {
+	curveStepBefore as d3CurveStepBefore,
+	curveStepAfter as d3CurveStepAfter,
 	curveBasisClosed as d3CurveBasisClosed,
 	curveBasisOpen as d3CurveBasisOpen,
 	curveBasis as d3CurveBasis,
@@ -157,7 +159,9 @@ extend(ChartInternal.prototype, {
 			"natural": d3CurveNatural,
 			"linear-closed": d3CurveLinearClosed,
 			"linear": d3CurveLinear,
-			"step": d3CurveStep
+			"step": d3CurveStep,
+			"step-after": d3CurveStepAfter,
+			"step-before": d3CurveStepBefore
 		}[interpolation];
 	},