Control which tick marks / labels appear on x-axis in plotly?

You can use style to add the ticks:

p <- plot_ly(data = df, x = x, y = y, type = 'line') %>%
    layout(title = 'Example plot', xaxis = list(autotick = F, dtick = 6))
p

Here are some more examples: https://plot.ly/r/axes/