Left Align Source Annotation or Axis Title for Source Below X-Axis

Plotly R, MATLAB, Julia, Net Plotly R
2

I think you just need to add more to the bottom margin? This will left-align the annotation with-respect-to the y-axis (is that what you want?)

df <- data.frame(x = 1:10, y = 1:10)
plot_ly(data = df, x = ~x, y = ~y, height = 400, width = 600) %>%
    add_lines() %>%
    layout(title = 'Sample Chart',
           margin = list(l = 50, r = 50, t = 60, b = 100),
           annotations = list(text = 'Source: U.S. Census Bureau.',
                              font = list(size = 12),
                              showarrow = FALSE,
                              xref = 'paper', x = 0,
                              yref = 'paper', y = -0.3))

PS. when using normalized (i.e., paper) coordinates, it’s a good idea to explicitly set the height/width, so the result is always the same, no matter the printing context.