Return null from getSelection if there's no selection.

Fixed pie plugin 100% slice issues on IE7/8.

When a slice equals (or comes close enough that it is equal in
floating-point representation) 100% of the pie, it was no longer drawn
by Excanvas.  This is because of a long-standing issue where VML won't
draw an arc whose begin and end are the same.  Rather than some hack,
like adding/subtracting a small value, we'll simply draw two arcs for
every slice, with each covering half of the slice's angle.

Fix tickDecimals option broken by pull #50.

Pull request #50 inadvertently broke the behavior of axis.tickDecimals,
which previously added precision up to the given value.  The broken code
effectively ignored the setting for values with less precision.  This
fix brings back the old behavior.

It's useful to include the tickDecimals option in one of our examples,
so it's easier to test across versions.  The setting-options example is
the logical place to put it.

Tested, verified and fixed support for jQuery 1.8.

Substitute zero when .width() returns undefined, since legendWidth is
used in numeric calculations that break when it is undefined.  This
became a problem with the release of jQuery 1.8.0, which returns
undefined instead of null when an element doesn't exist.  This was fixed
in 1.8.1, but this is good practice anyway.

Use placeholder dimensions instead og canvas dimensions in calculations which fixes draw bug after introducing Retina support in Flot core

Removed implicit global variables in threshold plugin.

Cleaner and portable function to get pixelratio.

This patch is intended the make getPixelRatio function portable and cleaner :)

Previous indentation issue has also been fixed.

Thanks :)

Support different border color and widths for each side of the chart.

Edited grammatical error.

Added Szymon Barglowski, who originally reported the fmt variable.

If we've called `clearSelection`, then calling `getSelection` shouldn't
return the old selection values. When calling `getSelection` before a
selection is made, `null` is returned. It seemed appropriate to do the
same if `selection.show` was falsy.
Signed-off-by: Nick Campbell <nicholas.j.campbell@gmail.com>

Prevent insertSteps and fmt from becoming global.

`fmt` was being hoisted as a global variable.
Signed-off-by: Nick Campbell <nicholas.j.campbell@gmail.com>