Here are the steps to reproduce the behavior I see:

1. Create a chart instance, but do not render it
2. Modify the chart dataProvider attribute
3. Render the chart. The original dataProvider attribute is the one used by the chart, the new dataProvider is ignored.

I expect the following two pieces of code to render the same thing:

YUI().use('charts', function (Y) {

        var cfg = {dataProvider: 0,1,2,3], [0,-1,-4,-16?};

        var c = Y.Chart(cfg);

        c.set('dataProvider', 0,1,2,3,4], [0,1,4,9,16?);

        c.render('#mychart');

    });

and

YUI().use('charts', function (Y) {

        var cfg = {dataProvider: 0,1,2,3], [0,-1,-4,-16?};

        var c = Y.Chart(cfg);

        c.render('#mychart');

        c.set('dataProvider', 0,1,2,3,4], [0,1,4,9,16?);

    });