mysql no data，why？

[hope1234567hope]

my yaml is

variables:
mysql_connection: mysql -u root -s --database mysql --skip-column-names
sparklines:

• title: MySQL (random number example)
  pty: true
  init: $mysql_connection
  sample: select rand();
  runcharts:
• title: thread moniot
  rate-ms: 2000
  items:
  • label: bgthread
    init: $mysql_connection
    sample: SELECT COUNT(*)
    FROM performance_schema.threads
    WHERE TYPE = 'BACKGROUND'
    AND NAME LIKE '%io%';
    color: 178

But does not display the corresponding values; it shows "NAN" instead.