0👍
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Your code expects different values for ctx
in the main code vs. your extension. In your main code it seems like a DOM node and in your extension you are using it as a context.
Just add
var ctx = this.chart.ctx;
after Chart.types.Line.prototype.draw.apply(this, arguments);
and it should work. If not, check if your main code variables are all defined with correct values (start
, end
and ctx
)