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This might not help, but my best mockups have been done on a half sheet of paper, or on a whiteboard. The beauty of Tableau is that it kind of is it's own mockup space - if you have charts already built, you can start throwing them into a dashboard, take a step back, and see what you need to re-arrange, what needs to be added, what needs to go away, etc.
Although... I'm interested to hear what others have to say about this.
I've used Balsamiq, Moqups, and Excel.
The easiest ? (as Ann said) A whiteboard or a piece of paper if Tableau is not available.