fix(xychart/Tooltip): enable style overflow in crosshairs

[williaster]

🐛 Bug Fix

Fixes #990. Previously setting strokeWidth on Tooltip crosshair styles had no effect because the parent SVG width/height of 1px didn't allow it. Fixed by setting overflow: visible.

Before
for vertical/horizontalCrosshairStyle={{ strokeWidth: 8, strokeDasharray: '4,4', stroke: 'yellow' }}

After
for same styles

@hshoff
cc @Janpot

[coveralls]

Pull Request Test Coverage Report for Build 467107801

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 61.196%

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Totals
Change from base Build 466988844:	0.0%
Covered Lines:	1728
Relevant Lines:	2690

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💛 - Coveralls