By default, X-axis label is “day X”. But, if you shrink this window width to smaller than 600px, the labels will changed to “week X”.

library(chartist)

set.seed(324)
data <- data.frame(
  day = 1:10,
  A   = runif(10, 0, 10),
  B   = runif(10, 0, 10),
  C   = runif(10, 0, 10)
)

chartist(data, day) +
  # By default, the axis labels are day-bases
  Line(x_labelInterpolationFnc = JS_interp(interval = 1, prefix = "day")) +
  # For small screens, they are week-bases
  Line(x_labelInterpolationFnc = JS_interp(interval = 7, prefix = "week"),
       showPoint = FALSE,
       responsiveQuery = "screen and (max-width: 600px)")

chartist(data, day) +
  # By default, draw a normal bar chart
  Bar(stackBars = FALSE) +
  # For smaller screens, draw a stacked bar chart
  Bar(stackBars = TRUE, responsiveQuery = "screen and (max-width: 600px)")

chartist(data[1:4, ], day) +
  # by default, draw a donut chart
  Pie(donut = TRUE, donutWidth = 100) +
  # for smaller screens, draw a normal pie chart
  Pie(donut = FALSE, showLabel = FALSE, responsiveQuery = "screen and (max-width: 600px)")