Moving chart loading spinner to chart component

fixes #885

frontend/src/app/components/statistics/statistics.component.html

@@ -1,14 +1,7 @@
 <div class="container-graph">
   <div>
-    <div *ngIf="loading" class="loading">
-      <div class="text-center">
-        <h3 i18n="statistics.loading-graphs">Loading graphs...</h3>
-        <br>
-        <div class="spinner-border text-light"></div>
-      </div>
-    </div>
     <div>
-      <div class="card mb-3" *ngIf="mempoolStats.length">
+      <div class="card mb-3">
         <div class="card-header">
           <i class="fa fa-area-chart"></i> <span i18n="statistics.memory-by-vBytes">Mempool by vBytes (sat/vByte)</span>
           <form [formGroup]="radioGroupForm" class="formRadioGroup" (click)="saveGraphPreference()">
@@ -80,7 +73,7 @@
               <button (click)="invertGraph()" class="btn btn-primary btn-sm"><fa-icon [icon]="['fas', 'exchange-alt']" [rotate]="90" [fixedWidth]="true" i18n-title="statistics.component-invert.title" title="Invert"></fa-icon></button>
             </div>
           </form>
-          <div class="spinner-border text-light bootstrap-spinner" *ngIf="spinnerLoading"></div>
+          <div class="spinner-border text-light bootstrap-spinner" *ngIf="spinnerLoading && mempoolStats.length"></div>
         </div>
         <div class="card-body">
           <div class="incoming-transactions-graph">
@@ -92,7 +85,7 @@
               [height]="500"
               [left]="65"
               [right]="10"
-              [data]="mempoolStats"
+              [data]="mempoolStats && mempoolStats.length ? mempoolStats : null"
             ></app-mempool-graph>
           </div>
         </div>
@@ -100,7 +93,7 @@
     </div>
 
     <div>
-      <div class="card mb-3" *ngIf="mempoolTransactionsWeightPerSecondData">
+      <div class="card mb-3">
         <div class="card-header">
             <i class="fa fa-area-chart"></i> <span i18n="statistics.transaction-vbytes-per-second">Transaction vBytes per second (vB/s)</span>
         </div>