Plotly Subplot Titles

library(plotly)
## Loading required package: ggplot2
## Warning: package 'ggplot2' was built under R version 3.3.2
## 
## Attaching package: 'plotly'
## The following object is masked from 'package:ggplot2':
## 
##     last_plot
## The following object is masked from 'package:stats':
## 
##     filter
## The following object is masked from 'package:graphics':
## 
##     layout
m <- economics[which.max(economics$unemploy), ]
n <- economics[which.max(economics$uempmed), ]

# font style
f <- list(
  family = "Courier New, monospace",
  size = 18,
  color = "black")

# annotations
a <- list(
  text = "SUBPLOT TITLE A",
  font = f,
  xref = "paper",
  yref = "paper",
  yanchor = "bottom",
  xanchor = "center",
  align = "center",
  x = 0.5,
  y = 1,
  showarrow = FALSE
)

b <- list(
  text = "SUBPLOT TITLE B",
  font = f,
  xref = "paper",
  yref = "paper",
  yanchor = "bottom",
  xanchor = "center",
  align = "center",
  x = 0.5,
  y = 1,
  showarrow = FALSE
)

p1 <- plot_ly(economics, x = ~date, y = ~unemploy) %>%
  add_lines(name = ~"unemploy") %>%
  layout(annotations = a)
p2 <- plot_ly(economics, x = ~date, y = ~uempmed) %>%
  add_lines(name = ~"uempmed") %>%
  layout(annotations = b)
p <- subplot(p1, p2, titleX = TRUE, titleY = TRUE) %>%
  layout(showlegend = FALSE)
p

Reference

See https://plot.ly/r/reference/#layout-annotations for more information and chart attribute options!