graph: initial histogram support #600 (#8053)

* graph: initial histogram support #600

* graph histogram mode: add Bars number option

* graph histogram mode: fix X axis ticks calculation

* graph histogram mode: change bar style (align and width)

* refactor(graph): move histogram functions into separate module

* graph histogram mode: rename series to "count"

* graph histogram mode: fix errors if no data

* refactor(graph and heatmap): move shared code into app/core

* graph: add tests for histogram mode

public/app/core/utils/ticks.ts

@@ -0,0 +1,27 @@
+/**
+ * Calculate tick step.
+ * Implementation from d3-array (ticks.js)
+ * https://github.com/d3/d3-array/blob/master/src/ticks.js
+ * @param start Start value
+ * @param stop End value
+ * @param count Ticks count
+ */
+export function tickStep(start: number, stop: number, count: number): number {
+  let e10 = Math.sqrt(50),
+    e5 = Math.sqrt(10),
+    e2 = Math.sqrt(2);
+
+  let step0 = Math.abs(stop - start) / Math.max(0, count),
+    step1 = Math.pow(10, Math.floor(Math.log(step0) / Math.LN10)),
+    error = step0 / step1;
+
+  if (error >= e10) {
+    step1 *= 10;
+  } else if (error >= e5) {
+    step1 *= 5;
+  } else if (error >= e2) {
+    step1 *= 2;
+  }
+
+  return stop < start ? -step1 : step1;
+}