Trace plot (or traceplot) of MCMC draws. See the Plot Descriptions section, below, for details.

mcmc_trace(x, pars = character(), regex_pars = character(),
  transformations = list(), facet_args = list(), ..., n_warmup = 0,
  window = NULL, size = NULL, np = NULL, np_style = trace_style_np(),
  divergences = NULL)

mcmc_trace_highlight(x, pars = character(), regex_pars = character(),
  transformations = list(), facet_args = list(), ..., n_warmup = 0,
  window = NULL, size = NULL, alpha = 0.2, highlight = 1)

trace_style_np(div_color = "red", div_size = 0.25, div_alpha = 1)



A 3-D array, matrix, list of matrices, or data frame of MCMC draws. The MCMC-overview page provides details on how to specify each these allowed inputs.


An optional character vector of parameter names. If neither pars nor regex_pars is specified then the default is to use all parameters.


An optional regular expression to use for parameter selection. Can be specified instead of pars or in addition to pars.


Optionally, transformations to apply to parameters before plotting. If transformations is a function or a single string naming a function then that function will be used to transform all parameters. To apply transformations to particular parameters, the transformations argument can be a named list with length equal to the number of parameters to be transformed. Currently only univariate transformations of scalar parameters can be specified (multivariate transformations will be implemented in a future release). If transformations is a list, the name of each list element should be a parameter name and the content of each list element should be a function (or any item to match as a function via, e.g. a string naming a function). If a function is specified by its name as a string (e.g. "log"), then it can be used to construct a new parameter label for the appropriate parameter (e.g. "log(sigma)"). If a function itself is specified (e.g. log or function(x) log(x)) then "t" is used in the new parameter label to indicate that the parameter is transformed (e.g. "t(sigma)").


A named list of arguments (other than facets) passed to facet_wrap or facet_grid to control faceting.


Currently ignored.


An integer; the number of warmup iterations included in x. The default is n_warmup = 0, i.e. to assume no warmup iterations are included. If n_warmup > 0 then the background for iterations 1:n_warmup is shaded gray.


An integer vector of length two specifying the limits of a range of iterations to display.


An optional value to override the default line size (mcmc_trace) or the default point size (mcmc_trace_highlight).


For models fit using NUTS (more generally, any symplectic integrator), an optional data frame providing NUTS diagnostic information. The data frame should be the object returned by nuts_params or one with the same structure. If np is specified then tick marks are added to the bottom of the trace plot indicating within which iterations there was a divergence (if there were any). See the end of the Examples section, below.


A call to the trace_style_np helper function to specify arguments controlling the appearance of tick marks representing divergences (if the np argument is specified).


Deprecated. Use the np argument instead.


For mcmc_trace_highlight, passed to geom_point to control the transparency of the points for the chains not highlighted.


For mcmc_trace_highlight, an integer specifying one of the chains that will be more visible than the others in the plot.

div_color, div_size, div_alpha

Optional arguments to the trace_style_np helper function that are eventually passed to geom_rug if the np argument is also specified. They control the color, size, and transparency specifications for showing divergences in the plot. The default values are displayed in the Usage section above.


A ggplot object that can be further customized using the ggplot2 package.

Plot Descriptions


Standard trace plots of MCMC draws. For models fit using NUTS, the np argument can be used to also show divergences on the trace plot.


Traces are plotted using points rather than lines and the opacity of all chains but one (specified by the highlight argument) is reduced.

See also

Other MCMC: MCMC-combos, MCMC-diagnostics, MCMC-distributions, MCMC-intervals, MCMC-nuts, MCMC-overview, MCMC-parcoord, MCMC-recover, MCMC-scatterplots


# some parameter draws to use for demonstration x <- example_mcmc_draws(chains = 4, params = 6) dim(x)
#> [1] 250 4 6
#> $Iteration #> NULL #> #> $Chain #> [1] "chain:1" "chain:2" "chain:3" "chain:4" #> #> $Parameter #> [1] "alpha" "sigma" "beta[1]" "beta[2]" "beta[3]" "beta[4]" #>
# trace plots of the betas color_scheme_set("viridis") mcmc_trace(x, regex_pars = "beta")
color_scheme_set("viridisA") mcmc_trace(x, regex_pars = "beta")
color_scheme_set("viridisC") mcmc_trace(x, regex_pars = "beta")
# mix color schemes color_scheme_set("mix-blue-red") mcmc_trace(x, regex_pars = "beta")
# use traditional ggplot discrete color scale mcmc_trace(x, pars = c("alpha", "sigma")) + ggplot2::scale_color_discrete()
#> Scale for 'colour' is already present. Adding another scale for 'colour', #> which will replace the existing scale.
# zoom in on a window of iterations, increase line size, # add tick marks, move legend to the top, add gray background color_scheme_set("viridisA") mcmc_trace(x[,, 1:4], window = c(100, 130), size = 1) + panel_bg(fill = "gray90", color = NA) + legend_move("top")
# NOT RUN { # parse facet label text color_scheme_set("purple") p <- mcmc_trace( x, regex_pars = "beta\\[[1,3]\\]", facet_args = list(labeller = ggplot2::label_parsed) ) p + facet_text(size = 15) # mark first 100 draws as warmup mcmc_trace(x, n_warmup = 100) # plot as points, highlighting chain 2 color_scheme_set("brightblue") mcmc_trace_highlight(x, pars = "sigma", highlight = 2, size = 2) # for models fit using HMC/NUTS divergences can be displayed in the trace plot library("rstanarm") fit <- stan_glm(mpg ~ ., data = mtcars, # next line to keep example fast and also ensure we get some divergences prior = hs(), iter = 400, adapt_delta = 0.8) # extract draws using as.array (instead of as.matrix) to keep # chains separate for trace plot posterior <- as.array(fit) # for stanfit and stanreg objects use nuts_params() to get the divergences mcmc_trace(posterior, pars = "sigma", np = nuts_params(fit)) color_scheme_set("viridis") mcmc_trace( posterior, pars = c("wt", "sigma"), size = 0.5, facet_args = list(nrow = 2), np = nuts_params(fit), np_style = trace_style_np(div_color = "black", div_size = 0.5) ) color_scheme_set("viridis") mcmc_trace( posterior, pars = c("wt", "sigma"), size = 0.8, facet_args = list(nrow = 2), divergences = nuts_params(fit), div_color = "black" ) # }