Step 1:Downloaded highcharts extension and extract it in /vendor/miloschuman/highcharts
Step 2: added following code in extension.php
    'miloschuman/highcharts' => array (
    'name' => 'miloschuman/highcharts',
    `enter code here`   'version' => '4',
    'alias' => array (
        '@miloschuman/highcharts' => $vendorDir . 'miloschuman/highcharts',
    ),
   ),
step3: added code in autoload_psr4.php
'miloschuman\\highcharts\\' => array($vendorDir . '/miloschuman/highcharts'),
Step 4 :in view
use miloschuman\highcharts\Highcharts;
but getting error
Class 'miloschuman\highcharts\Highcharts' not found
                        
Please visit https://github.com/miloschuman/yii2-highcharts
It worked fine for me. Hope this work fine for you also.
You can download the ZIP file and follow the process accordingly as given in the link.