Regarding diplaying form once

I think you have a confusion between form classes and instances. When you say that something is shown "twice", it may only mean that you have two instances of the same class at the same time. If you need to avoid it, you should work with only one instance. There are a number of ways to do it, but the best and most universal approach here is using the lazy pattern, in it simplest form:

partial class MyMainForm {

    MyChartForm chartForm; // at first, it's null; will remain null before first use

    void ShowChart() { //add "internal" if you want to use it from outside the form
       if (chartForm == null) {
           chartForm = new MyChartForm();
           chartForm.Owner = this;
           chartForm.ShowInTaskbar = false; // important for application integrity
       } //if chartForm == null
       chartForm.ShowDialog();
    } //ShowChart

} //class MyMainForm

One benefit of this approach is that you are not risking of creating a form too early, when some of the operations would be invalid. For System.Windows.Forms, this is not a big problem, but with WPF, it can really save a day.

This technique will require something else if you want to have a non-modal additional form. [EDIT] Such situation appears when Form.Show method is used instead of Form.ShowDialog. [END EDIT]

In this case, I would advise you to prevent closing (as you won't be able to show it again because of disposed objects; you need to hide it on FormClosing and cancel closing (this event can be canceled; the event argument type has Cancel Boolean member; you will need to set it to true), to show the form again later.

Please see also:
http://msdn.microsoft.com/en-us/library/system.componentmodel.canceleventargs.aspx[^],
http://en.wikipedia.org/wiki/Lazy_initialization[^].

—SA