jQuery: How to keep event handlers when removing custom ones

Question

I have created a simple drawing script using canvas: http://ulrichbangert.de/div/webentwicklung/javascript/jcanvas-drawing-pad.html I need event handlers and I attach them like this (very simplyfied):

<canvas id="mycanvas" width="400" height="300"></canvas>
<script>
    var line = {
        activate: function () {
            $("#mycanvas").off();
            $("#mycanvas").on("mousemove", function (e) {
                // do something to draw line
            })
        }
    }
    var text = {
        activate: function () {
            $("#mycanvas").off();
            $("#mycanvas").on("mousemove", function (e) {
                // do something to draw text
            })
        }
    }
    line.activate();
    // later on:
    text.activate();
</script>

The problem is that I'm using the library jCanvas that attaches eventhandlers too. By

            $("#mycanvas").off();

I remove all handlers including those attached by jCanvas, so that some features don't work. How can I remove the handlers attached by me and keep the library's handlers intact?