persist graph window preference between reloads (#456)

Persist graph window preference between reloads
fixes #365

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

@@ -14,8 +14,7 @@
       <div class="card mb-3" *ngIf="mempoolStats.length">
         <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" style="float: right;">
+          <form [formGroup]="radioGroupForm" style="float: right;" (click)='saveGraphPreference()'>
             <div class="spinner-border text-light bootstrap-spinner" *ngIf="spinnerLoading"></div>
             <div class="btn-group btn-group-toggle" ngbRadioGroup name="radioBasic" formControlName="dateSpan">
               <label ngbButtonLabel class="btn-primary btn-sm">