I cannot reproduce your issue using JMeter 5.2.1 and Graphs Generator Listener 2.0, here is listing of my JMeter's "lib" folder:
C:\TEMP\APACHE-JMETER-5.2.1\LIB
│ accessors-smart-1.2.jar
│ apiguardian-api-1.1.0.jar
│ asm-7.1.jar
│ bsf-2.4.0.jar
│ bsh-2.0b6.jar
│ bshclient.jar
│ caffeine-2.8.0.jar
│ checker-qual-2.10.0.jar
│ commons-codec-1.13.jar
│ commons-collections-3.2.2.jar
│ commons-dbcp2-2.5.0.jar
│ commons-io-2.6.jar
│ commons-jexl-2.1.1.jar
│ commons-jexl3-3.1.jar
│ commons-lang3-3.9.jar
│ commons-logging-1.2.jar
│ commons-math3-3.6.1.jar
│ commons-net-3.6.jar
│ commons-pool2-2.7.0.jar
│ commons-text-1.8.jar
│ darcula-e208efb96f70e4be9dc362fbb46f6e181ef501dd.jar
│ dec-0.1.2.jar
│ dnsjava-2.1.9.jar
│ error_prone_annotations-2.3.3.jar
│ freemarker-2.3.29.jar
│ geronimo-jms_1.1_spec-1.1.1.jar
│ groovy-all-2.4.16.jar
│ hamcrest-2.1.jar
│ hamcrest-core-2.1.jar
│ hamcrest-date-2.0.4.jar
│ httpasyncclient-4.1.4.jar
│ httpclient-4.5.10.jar
│ httpcore-4.4.12.jar
│ httpcore-nio-4.4.12.jar
│ httpmime-4.5.10.jar
│ jackson-annotations-2.9.10.jar
│ jackson-core-2.9.10.jar
│ jackson-databind-2.9.10.jar
│ javax.activation-1.2.0.jar
│ jcharts-0.7.5.jar
│ jcl-over-slf4j-1.7.28.jar
│ jmespath-core-0.3.0.jar
│ jmespath-jackson-0.3.0.jar
│ jmeter-plugins-cmn-jmeter-0.4.jar
│ jodd-core-5.0.13.jar
│ jodd-lagarto-5.0.13.jar
│ jodd-log-5.0.13.jar
│ jodd-props-5.0.13.jar
│ jorphan.jar
│ json-path-2.4.0.jar
│ json-smart-2.3.jar
│ jsoup-1.12.1.jar
│ jtidy-r938.jar
│ junit-4.12.jar
│ log4j-1.2-api-2.12.1.jar
│ log4j-api-2.12.1.jar
│ log4j-core-2.12.1.jar
│ log4j-slf4j-impl-2.12.1.jar
│ mail-1.5.0-b01.jar
│ miglayout-core-5.2.jar
│ miglayout-swing-5.2.jar
│ mongo-java-driver-2.11.3.jar
│ neo4j-java-driver-1.7.5.jar
│ oro-2.0.8.jar
│ ph-commons-9.3.7.jar
│ ph-css-6.2.0.jar
│ rhino-1.7.11.jar
│ rsyntaxtextarea-3.0.4.jar
│ Saxon-HE-9.9.1-5.jar
│ serializer-2.7.2.jar
│ slf4j-api-1.7.28.jar
│ tika-core-1.22.jar
│ tika-parsers-1.22.jar
│ xalan-2.7.2.jar
│ xercesImpl-2.12.0.jar
│ xml-apis-1.4.01.jar
│ xmlgraphics-commons-2.3.jar
│ xmlpull-1.1.3.1.jar
│ xpp3_min-1.1.4c.jar
│ xstream-1.4.11.jar
│
├───ext
│ ApacheJMeter_bolt.jar
│ ApacheJMeter_components.jar
│ ApacheJMeter_core.jar
│ ApacheJMeter_ftp.jar
│ ApacheJMeter_functions.jar
│ ApacheJMeter_http.jar
│ ApacheJMeter_java.jar
│ ApacheJMeter_jdbc.jar
│ ApacheJMeter_jms.jar
│ ApacheJMeter_junit.jar
│ ApacheJMeter_ldap.jar
│ ApacheJMeter_mail.jar
│ ApacheJMeter_mongodb.jar
│ ApacheJMeter_native.jar
│ ApacheJMeter_tcp.jar
│ jmeter-plugins-graphs-ggl-2.0.jar
│ jmeter-plugins-manager-0.20.jar
│ readme.txt
│
└───junit
readme.txt
The listing was generated using tree /F apache-jmeter-5.2.1\lib command, inspect your own folder and make sure it's in sync with what's displayed above.
Normally the reason of any JMeter failure or at least a clue can be found in jmeter.log file so take a look there - most probably you will see what's causing the error.
With regards to the Plugins Manager - you should be able to use it behind the proxy, it respects JMeter Proxy configuration, check out Plugins Manager Network Configuration chapter for more troubleshooting steps