library(plotly)

fig <- plot_ly(
   type = 'scatter',
   mode='markers',
   y=rep(5, 40),
   marker=list(
      size=seq(0, 39),
      color=seq(0, 39),
      colorbar=list(
         title='Colorbar'
      ),
      colorscale='Viridis',
      reversescale =F
   )
)

fig <- fig %>% layout(
   xaxis = list(
      showgrid = F,
      zeroline = F
   ),
   yaxis = list(
      showgrid = F,
      zeroline = F
   )
)

fig