Create difficulty chart component

frontend/src/app/components/difficulty-chart/difficulty-chart.component.html

@@ -0,0 +1,8 @@
+<div class="container-xl">
+
+  <div *ngIf="difficultyObservable$ | async" class="" echarts [initOpts]="chartInitOptions" [options]="chartOptions"></div>
+  <div class="text-center loadingGraphs" *ngIf="isLoading">
+    <div class="spinner-border text-light"></div>
+  </div>
+
+</div>

frontend/src/app/components/difficulty-chart/difficulty-chart.component.ts

@@ -0,0 +1,102 @@
+import { Component, OnInit } from '@angular/core';
+import { EChartsOption } from 'echarts';
+import { Observable } from 'rxjs';
+import { map, tap } from 'rxjs/operators';
+import { ApiService } from 'src/app/services/api.service';
+import { SeoService } from 'src/app/services/seo.service';
+
+@Component({
+  selector: 'app-difficulty-chart',
+  templateUrl: './difficulty-chart.component.html',
+  styleUrls: ['./difficulty-chart.component.scss'],
+  styles: [`
+    .loadingGraphs {
+      position: absolute;
+      top: 38%;
+      left: calc(50% - 15px);
+      z-index: 100;
+    }
+  `],
+})
+export class DifficultyChartComponent implements OnInit {
+  chartOptions: EChartsOption = {};
+  chartInitOptions = {
+    renderer: 'svg'
+  };
+
+  difficultyObservable$: Observable<any>;
+  isLoading = true;
+
+  constructor(
+    private seoService: SeoService,
+    private apiService: ApiService,
+  ) {
+    this.seoService.setTitle($localize`:@@mining.difficulty:Difficulty`);
+  }
+
+  ngOnInit(): void {
+    this.difficultyObservable$ = this.apiService.getHistoricalDifficulty$(undefined)
+      .pipe(
+        map(data => {
+          return data.map(val => [val.timestamp, val.difficulty])
+        }),
+        tap(data => {
+          this.prepareChartOptions(data);
+          this.isLoading = false;
+        })
+      )
+  }
+
+  prepareChartOptions(data) {
+    this.chartOptions = {
+      title: {
+        text: $localize`:@@mining.difficulty:Difficulty`,
+        left: 'center',
+        textStyle: {
+          color: '#FFF',
+        },
+      },
+      tooltip: {
+        show: true,
+        trigger: 'axis',
+      },
+      axisPointer: {
+        type: 'line',
+      },
+      xAxis: [
+        {
+          type: 'time',
+        }
+      ],
+      yAxis: {
+        type: 'value',
+        axisLabel: {
+          fontSize: 11,
+          formatter: function(val) {
+            const diff = val / Math.pow(10, 12); // terra
+            return diff.toString() + 'T';
+          }
+        },
+        splitLine: {
+          lineStyle: {
+            type: 'dotted',
+            color: '#ffffff66',
+            opacity: 0.25,
+          }
+        }
+      },
+      series: [
+        {
+          data: data,
+          type: 'line',
+          smooth: false,
+          lineStyle: {
+            width: 3,
+          },
+          areaStyle: {}
+        },
+      ],
+    };
+  }
+
+}

frontend/src/app/components/pool-ranking/pool-ranking.component.ts

@@ -1,4 +1,4 @@
-import { Component, OnDestroy, OnInit } from '@angular/core';
+import { Component, OnInit } from '@angular/core';
 import { FormBuilder, FormGroup } from '@angular/forms';
 import { Router } from '@angular/router';
 import { EChartsOption, PieSeriesOption } from 'echarts';
@@ -23,7 +23,7 @@ import { StateService } from '../../services/state.service';
     }
   `],
 })
-export class PoolRankingComponent implements OnInit, OnDestroy {
+export class PoolRankingComponent implements OnInit {
   poolsWindowPreference: string;
   radioGroupForm: FormGroup;
 
@@ -90,9 +90,6 @@ export class PoolRankingComponent implements OnInit, OnDestroy {
       );
   }
 
-  ngOnDestroy(): void {
-  }
-
   formatPoolUI(pool: SinglePoolStats) {
     pool['blockText'] = pool.blockCount.toString() + ` (${pool.share}%)`;
     return pool;