update plotType drop down on selection

The drop down for plot types should only contain plot types that can
be combined. The reason is, that we can only draw images with two
x/y-axis. Therefore a combination of types that would need three or more
axis is not supported.

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

@@ -17,7 +17,7 @@
 
       <mat-form-field>
         <mat-label>Type:</mat-label>
-        <mat-select multiple [(value)]="selectedPlotType">
+        <mat-select multiple [(ngModel)]="selectedPlotType" (ngModelChange)="changePlotType($event)">
           <mat-option *ngFor="let plotType of plotTypes" [value]="plotType">
             <img src="assets/img/{{plotType.icon}}.svg" class="icon-select" /> {{plotType.name}}
           </mat-option>