Plot daily data consumption calculated by vnstat using gnuplot's bar plot

plot.gnuplot

set style fill solid 0.5;
set xtics rotate
set xlabel "Day";
set ylabel "MiB";

# data file passed as a command-line argument (variable ARG1)
# 0: pseudo-column for indexes starting from zero = x-axis
# 2: y-axis=2nd column in file
# xticklabels(1): ticks on x-axis from 1st column in file
plot ARG1 using 0:2:xticlabels(1) title "Daily data usage in MiB" with boxes;

# wait till <Enter> key is hit
pause -1

plot.sh

#!/usr/bin/env bash
set -e

if [ "$#" -ne 1 ]; then
  echo "USAGE: $0 NETWORK_INTERFACE"
  exit 1
fi

# jq used only for indenting output
data_file="/tmp/data.json"
n_days_max="30"
vnstat --json d "$n_days_max" | jq '.' > "$data_file"

# first network interface not always the one used
interface="$1"
traffic_file="/tmp/traffic.json"
jq ".interfaces | map(select(.name == \"$interface\")).[0] | .traffic.day" "$data_file" > "$traffic_file"

# handles case when vnstat was just installed (not enough data)
n_days=$(jq 'length' "$traffic_file")

txt_file="/tmp/data.txt"
truncate -s 0 "$txt_file"

for (( i = 0; i < "$n_days"; i++ )); do
  # get date (x-axis)
  year=$(jq ".[$i].date.year" "$traffic_file")
  month=$(jq ".[$i].date.month" "$traffic_file")
  day=$(jq ".[$i].date.day" "$traffic_file")

  # pad days & months with zero
  day=$(printf "%02u" "$day")
  month=$(printf "%02u" "$month")
  date_day="$year-$month-$day"

  # convert bytes to MiB & round-up with printf (y-axis)
  rx=$(jq ".[$i].rx" "$traffic_file")
  rx_mib=$(echo "scale=3; $rx / 2^20" | bc)
  rx_mib=$(printf "%.2f" "$rx_mib")

  echo "$date_day $rx_mib" >> "$txt_file"
done

# ticks labels taken from 1st column (i.e. dates)
gnuplot -c "plot.gnuplot" "$txt_file"