mysql no data，why？

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.