igloo: place the pin by clicking the graph

Usability testing showed users expect to set the pin by clicking the
plot, not the handle above it. A sub-threshold press now pins the
sample under the cursor, reusing the threshold that already told a
click from a zoom drag. The + / - glyphs become the design's teardrop
marker, which stands in the gap above the plot rather than taking plot
height, and dashboard graph widgets honour their existing "Click to
place pin" option.

JIRA-Ref: CMK-38050
Change-Id: Ic3444a4d01ec9f9cc2acacc4afbd6147c4b02c4f