use one multi-select drop down for plot type

All plot types we currently have can be combined
in one image. So there is no need to compute
combinable types.

pdb-js/src/app/visualization-page/visualization-page.component.html

@@ -17,23 +17,13 @@
 
       <mat-form-field>
         <mat-label>Type:</mat-label>
-        <mat-select [formControl]="selectedPlotType">
+        <mat-select multiple [(value)]="selectedPlotType">
           <mat-option *ngFor="let plotType of plotTypes" [value]="plotType">
             <img src="assets/img/{{plotType.icon}}.svg" class="icon-select" /> {{plotType.name}}
           </mat-option>
         </mat-select>
       </mat-form-field>
     
-      <mat-form-field>
-        <mat-label>Combine With Type:</mat-label>
-        <mat-select [formControl]="selectedCombinePlotType">
-          <mat-option>-</mat-option>
-          <mat-option *ngFor="let plotType of combinePlotTypes" [value]="plotType">
-            <img src="assets/img/{{plotType.icon}}.svg" class="icon-select" /> {{plotType.name}}
-          </mat-option>
-        </mat-select>
-      </mat-form-field>
-    
       <mat-form-field>
         <mat-label>Group By:</mat-label>
         <mat-select multiple [(value)]="groupBy">