replace 'showzeroline' -> 'zeroline'
replace 'showzeroline' -> 'zeroline'

- 'showzeroline' isn't a valid a attribute
- the tests worked fine as 'zeroline' got coerced to true
  in all the cases present.

plotly/plotly.js,etpinard/plotly.js,plotly/plotly.js,plotly/plotly.js,etpinard/plotly.js,etpinard/plotly.js,aburato/plotly.js,aburato/plotly.js,plotly/plotly.js