Fix for issue with log scale and toDataYCoord.

Fix for issue with log scale and toDataYCoord.

This is a fix for issue 454:
https://code.google.com/p/dygraphs/issues/detail?id=454

Squashed commit of the following:

commit 795eaa7bb3fcf65ae13c861dd8c034583009516e
Author: craigj <craig.jackson@douglasscientific.com>
Date:   Thu Aug 29 07:21:56 2013 -0500

    Adding back in comment that explains calculation that was removed in a previous commit.

commit fc837121be749eefc41c2af0f5b376dd593bedb6
Author: craigj <craig.jackson@douglasscientific.com>
Date:   Tue Aug 27 13:54:27 2013 -0500

    Adding test testChartLogarithmic to auto_test.

commit 3e1f6af6a79517fce4b88d59def0d05e4ac90920
Author: craigj <craig.jackson@douglasscientific.com>
Date:   Tue Aug 13 14:19:09 2013 -0500

    ISSUE-454:  Fixing issue with log scale and toDataYCoord per issue 454.  https://code.google.com/p/dygraphs/issues/detail?id=454

commit 7ea65b58b912714594f5f9382464167f34dfd03a
Author: craigj <craig.jackson@douglasscientific.com>
Date:   Tue Aug 13 14:16:59 2013 -0500

    ISSUE-454:  Fixing issue with log scale and toDataYCoord per issue 454.  https://code.google.com/p/dygraphs/issues/detail?id=454

commit d509655be656940deaec9a06cdab35c451e708d5
Author: craigj <craig.jackson@douglasscientific.com>
Date:   Tue Aug 13 14:11:35 2013 -0500

    ISSUE-454:  Fixing issue with log scale and toDataYCoord per issue 454.  https://code.google.com/p/dygraphs/issues/detail?id=454