Data frames are optional (especially important for trace types with 'z' argument)

[cpsievert]

The R reference reads as though a data frame is required (it isn't):

A histogram trace is initialized with plot_ly or add_trace:
plot_ly(df, type="histogram"[, ...])
add_trace(p, type="histogram"[, ...])

Also, for some traces types (e.g., heatmap, contour, histogram2d, histogram2dcontour), it's more natural to give 'z' a numeric matrix, so

z (dataframe column, list, vector)
Sets the z data.

Should say

z (numeric matrix)
Sets the z data.

This will fix plotly/plotly.R#329