Use ARIA label instead of alt text for plots

visualizer/static/visualizer.js

@@ -85,7 +85,7 @@ function visualize(imgSrc) {
   const visualization = document.getElementById('visualization');
   const img = document.createElement('img');
   img.src = imgSrc;
-  img.alt = 'Graph of selected data';
+  img.setAttribute('aria-label', 'Graph of selected data');
   plotDiv.replaceChildren(img);
   visualization.hidden = false;
 }

visualizer/templates/plotter.html

@@ -76,7 +76,7 @@
     <div id="plot">
     {%- if g.selection_complete +%}
       <img src="{{ url_for('plot_image', name=g.plotter.name) -}}
-                ?{{ g.image_qs }}" alt="Graph of selected data">
+                ?{{ g.image_qs }}" aria-label="Graph of selected data">
     {%+ endif -%}
     </div>
     <div id="sources">