whilst the answer is very good it is a little outdated now. Google has implemented CSS control so there is greater flexibility without the need to hack the JavaScript.
.google-visualization-tooltip { position:relative !important; top:0 !important;right:0 !important; z-index:+1;}
will provide a tooltip fixed at the bottom of the chart, live example: http://www.taxformcalculator.com/federal-budget/130000.html
alternatively you could just tweak the left margin...
.google-visualization-tooltip { margin-left: 150px !important; z-index:+1;}
Note that pulling the container forward with z-index reduces (but does not stop entirely) visibility flicker as the mouse moves. The degree of flicker will vary on chart size, call etc. Personally, I prefer to fix the tool tip and make it part of the design as per the first example. Hope this helps those who are deterred by the JS hack (which is good but really no longer necessary).
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