Probably you are right. GfxSetCoordsMode(1) doesn't seem to work with GuiButton() the way it works with other Gfx functions. Either it hasn't been implemented yet or Tomasz did it on purpose - not to let users abuse it for example to display buttons on every single candle 
But you can always do it the old way - converting between bars/price and pixels or just simplify it (bypassing the convertion) in many different ways.
If you are interested in dynamic buttons positioning, you can take a look at my Buttons Hunter Game for AB code, which bases exactly on that.
If someone says the game is easy, I encourage everyone to play it on Difficult or Insane level. Good luck 
I bet no one scores 100 points. If he/she succeeds and proves that by showing the screenshot, this user will have my respect
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