Fix highlighting for usage with cross-filter selection

[spren9er]

Consider following scenario:

data:
  athletes: { file: data/athletes.csv }
params:
  query: { select: crossfilter }
  click: { select: single }
vconcat:
- hconcat:
  - input: menu
    label: Sport
    as: $query
    from: athletes
    column: sport
- vspace: 20
- name: total
  plot:
  - mark: barX
    data: { from: athletes, filterBy: $query }
    x: { count: '' }
  xLabel: Total →
  xDomain: Fixed
- vspace: 20
- name: total_by_country
  plot:
  - mark: barY
    data: { from: athletes, filterBy: $query }
    x: nationality
    y: { count: '' }
    sort: { x: -y, limit: 10 }
  - select: toggleX
    as: $query
  - select: toggleX
    as: $click
  - select: highlight
    by: $click
  xLabel: ''
  yLabel: Total

As soon as a sport is selected in menu, highlighting does not work properly when clicking on bars.

Reason: Predicate function used for highlighting is based on query which is missing WHERE clause, e.g. here sport = 'athletics'.

See also #144 (reply in thread).

This PR covers a fix for above issue.

[jheer]

Thank you for identifying the crux of the issue. I made a slightly different fix, so this PR was superceded by #147.