CanvasJS Chart automatically sets the width and height of the chart according to container’s dimensions. When the container’s dimension is not specified it takes up the default width & height(500px X 400px). Setting 100% width for the Chart-Container will work fine in flexbox layout. Please take a look at this jsfiddle.
If this doesn’t solve the issue, kindly create a jsfiddle with the issue you are facing so that we can understand it better and help you out.