Let's try this:
1) Create a new text document and save it as Test.html
2) In the text document, add this:
<html> <iframe width="450" height="260" style="border: 1px solid #cccccc;" src="https://thingspeak.com/apps/matlab_visualizations/YourMATLABVizNumber"></iframe> </html>
3) Open this up in your browser. Does this look like you expect?
If this does not look like the plot you expect, confirm that the plot generated by your MATLAB visualization is added to your channel and the YourMATLABVisualizationNumber corresponds to the plot for your channel.