Made 'type' optional to avoid problems with Chartist defaults

When forced to define the type, Chartist won't default to all of the
other options.  So if you just want to do something like not show the
grid, you'll end up having to define all of the axis options because of
the strict typing that was there before.

chartist/chartist-tests.ts

@@ -242,6 +242,26 @@ new Chartist.Line('.ct-chart', {
     }
   });
 
+new Chartist.Line('.ct-chart', {
+  labels: [1, 2, 3, 4, 5, 6, 7, 8],
+  series: [
+    [5, 9, 7, 8, 5, 3, 5, 4]
+  ]
+}, {
+    ticks: [0, 4],
+    low: 0,
+    showArea: true,
+    axisX: {
+      showGrid: false
+    },
+    axisY: {
+      showLabel: true,
+      showGrid: false,
+      ticks: [1, 4],
+      type: Chartist.FixedScaleAxis
+    }
+  });
+
 var chart2 = new Chartist.Line('.ct-chart', {
   labels: [1, 2, 3, 4, 5],
   series: [

chartist/chartist.d.ts

@@ -305,13 +305,13 @@ declare module Chartist {
   }
 
   interface IChartistStepAxis extends ILineChartAxis {
-    type: IStepAxisStatic;
+    type?: IStepAxisStatic;
     ticks?: Array<string> | Array<number>;
     stretch?: boolean;
   }
 
   interface IChartistFixedScaleAxis extends ILineChartAxis {
-    type: IFixedScaleAxisStatic;
+    type?: IFixedScaleAxisStatic;
     high?: number;
     low?: number;
     divisor?: number;
@@ -324,7 +324,7 @@ declare module Chartist {
     scaleMinSpace?: number;
     onlyInteger?: boolean;
     referenceValue?: number;
-    type: IAutoScaleAxisStatic;
+    type?: IAutoScaleAxisStatic;
   }
 
   interface ILineChartClasses {